fix(arrow-json): validate map value nullability

[subotac]

AI disclosure: Codex assisted with issue analysis and running local test

Which issue does this PR close?

• Closes Reading json map with non-nullable value schema doesn't error if values are actually null #6391.

Rationale for this change

The JSON map decoder constructed map entries with an unchecked StructArray constructor. This bypassed child field nullability validation and allowed a null value even when the map value field was non-nullable.

What changes are included in this PR?

• Construct map entries with StructArray::try_new_with_length so existing schema validation rejects unmasked nulls.
• Add a regression test for a null JSON map value with a non-nullable value field.

Are these changes tested?

Yes:

• cargo +stable fmt --all -- --check
• cargo test -p arrow-json --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• git diff --check

Are there any user-facing changes?

Yes. Reading a null JSON map value against a non-nullable value field now returns an error instead of producing an array that violates its schema. There are no public API changes.

[Jefffrey]

thanks @subotac