fix(protobuf): tolerate empty custom fields

[richard-ramos]

Summary

Treat present-but-empty protobuf payloads for single-value custom MultiAddress, PeerId, PublicKey, and Signature fields as an unsuccessful field decode instead of raising from the custom decoder.

Non-empty malformed payloads still raise ProtobufValueError, preserving fatal decode behavior for corrupt field contents.

Affected Areas

• Protocol Logic
  • Custom protobuf field decoding for multiaddresses, peer IDs, public keys, and signatures.

Impact on Library Users

Optional fields encoded as empty bytes are now handled like absent field values at the field decoder level, so they do not abort the surrounding protobuf decode solely because the optional field is empty.

Risk Assessment

Low. Successful decoding is unchanged, and malformed non-empty values continue to fail as before.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.29%. Comparing base (057dfb3) to head (ef4de71).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
libp2p/peerid.nim	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2789      +/-   ##
==========================================
- Coverage   81.32%   81.29%   -0.03%     
==========================================
  Files         171      171              
  Lines       31013    31023      +10     
  Branches       12       12              
==========================================
  Hits        25221    25221              
- Misses       5792     5802      +10     

Files with missing lines	Coverage Δ
libp2p/crypto/crypto.nim	64.68% <100.00%> (+0.22%)	⬆️
libp2p/multiaddress.nim	85.58% <100.00%> (-0.17%)	⬇️
libp2p/peerid.nim	90.19% <50.00%> (-0.36%)	⬇️

... and 8 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.