fix: allow nullable owner in Nft record and handle null account_id for burned NFTs

[Twiineenock]

The Hedera mirror node returns "account_id": null for burned NFTs — this is correct,
documented behaviour. A burned NFT still exists as an on-chain record but has no owner.

Previously, MirrorNodeJsonConverterImpl.toNft() called
AccountId.fromString(node.get("account_id").asText()) without a null check, causing
AccountId.fromString("null") to throw, which propagated as a JsonParseException (500)
whenever a token's NFT list contained burned serial numbers.

Changes:

• Allow @Nullable AccountId owner in Nft.java — accurately models the Hedera data model
  where burned NFTs have no owner
• Add null check in Spring MirrorNodeJsonConverterImpl.toNft() before parsing account_id
• Add null check in Microprofile MirrorNodeJsonConverterImpl.toNft() for consistency

Related issue(s):

Fixes #237

Notes for reviewer:

The fix is consistent with how every other nullable account field is already handled in the
same files (e.g. receiver_account_id, collector_account_id all have null guards).
account_id on the NFT object was the only field missing this guard.

Screenshot

Checklist

• Documented (code comments explaining why account_id can be null)
• Tested (unit, integration, etc.)

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Walkthrough

The Nft record now permits null owners for burned NFTs. Both mirror-node JSON converters preserve null account_id values, and tests cover nullable construction, validation, and NFT JSON parsing.

Changes

Nullable NFT Owner Support

Layer / File(s)	Summary
Nft record contract and validation tests hiero-enterprise-base/src/main/java/org/hiero/base/data/Nft.java, hiero-enterprise-base/src/test/java/org/hiero/base/test/NftTest.java	owner is annotated as nullable, burned NFTs are documented, owner null validation is removed, and constructor/accessor validation is tested.
Mirror-node null owner parsing and coverage hiero-enterprise-microprofile/.../MirrorNodeJsonConverterImpl.java, hiero-enterprise-spring/.../MirrorNodeJsonConverterImpl.java, hiero-enterprise-spring/.../MirrorNodeJsonConverterImplTest.java	Both converters handle JSON-null account_id; tests cover live and burned NFTs, empty inputs, missing arrays, and null-node handling.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant MirrorNode
  participant MirrorNodeJsonConverterImpl
  participant Nft
  participant NftTest

  MirrorNode->>MirrorNodeJsonConverterImpl: NFT JSON with account_id null
  MirrorNodeJsonConverterImpl->>MirrorNodeJsonConverterImpl: Detect null account_id
  MirrorNodeJsonConverterImpl->>Nft: Construct Nft(owner=null)
  Nft-->>MirrorNodeJsonConverterImpl: Nullable Nft instance
  NftTest->>Nft: Validate nullable owner and constructor rules

Loading

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 13.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main fix: supporting nullable NFT owners and null account_id parsing for burned NFTs.
Description check	✅ Passed	The description is directly related to the changes and accurately describes the burned NFT null-handling fix.
Linked Issues check	✅ Passed	The changes address #237 by preventing null account_id parsing failures and adding coverage for burned NFTs.
Out of Scope Changes check	✅ Passed	The added nullable model change, parser updates, and tests all support the burned NFT fix and do not appear out of scope.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yml

Review profile: ASSERTIVE

Plan: Pro

Run ID: c367ee47-a04b-4b1a-93f1-d39faea3eb81

📥 Commits

Reviewing files that changed from the base of the PR and between ffa5603 and 5450510.

📒 Files selected for processing (2)

• hiero-enterprise-base/src/test/java/org/hiero/base/test/NftTest.java
• hiero-enterprise-spring/src/test/java/org/hiero/spring/test/MirrorNodeJsonConverterImplTest.java

[coderabbitai]

📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low value

Consider asserting metadata round-trip in the toNft tests.

Both toNft_withValidAccountId_returnsNftWithOwner and toNft_withNullAccountId_returnsNftWithNullOwner set METADATA_BASE64 in the JSON node but don't verify that nft.metadata() decodes back to the original METADATA bytes. The converter uses node.get("metadata").binaryValue() (base64 decode), so a regression in that decoding path would go undetected.

🧪 Suggested addition

     Assertions.assertEquals(SERIAL, nft.serial());
     Assertions.assertEquals(AccountId.fromString(ACCOUNT_ID), nft.owner());
+    Assertions.assertArrayEquals(METADATA, nft.metadata());
   }

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change

	@Test
	void toNft_withValidAccountId_returnsNftWithOwner() throws Exception {
	// given — a live NFT with a non-null account_id
	final ObjectNode node = objectMapper.createObjectNode();
	node.put("token_id", TOKEN_ID);
	node.put("account_id", ACCOUNT_ID);
	node.put("serial_number", SERIAL);
	node.put("metadata", METADATA_BASE64);
	// when
	final Optional<Nft> result = converter.toNft(node);
	// then
	Assertions.assertTrue(result.isPresent());
	final Nft nft = result.get();
	Assertions.assertEquals(TokenId.fromString(TOKEN_ID), nft.tokenId());
	Assertions.assertEquals(SERIAL, nft.serial());
	Assertions.assertEquals(AccountId.fromString(ACCOUNT_ID), nft.owner());
	}
	`@Test`
	void toNft_withValidAccountId_returnsNftWithOwner() throws Exception {
	// given — a live NFT with a non-null account_id
	final ObjectNode node = objectMapper.createObjectNode();
	node.put("token_id", TOKEN_ID);
	node.put("account_id", ACCOUNT_ID);
	node.put("serial_number", SERIAL);
	node.put("metadata", METADATA_BASE64);
	// when
	final Optional<Nft> result = converter.toNft(node);
	// then
	Assertions.assertTrue(result.isPresent());
	final Nft nft = result.get();
	Assertions.assertEquals(TokenId.fromString(TOKEN_ID), nft.tokenId());
	Assertions.assertEquals(SERIAL, nft.serial());
	Assertions.assertEquals(AccountId.fromString(ACCOUNT_ID), nft.owner());
	Assertions.assertArrayEquals(METADATA, nft.metadata());
	}