Fix HNSW k=0 query crash

[hoteye]

Description of changes

• Improvements & Bug fixes
  • Fix HNSW query crash when k == 0 (empty collection / fully filtered).
  • Skip persistent HNSW query when hnsw_k == 0.

Test plan

• Not run.

Migration plan

• None.

Observability plan

• None.

Documentation Changes

• None.

[github-actions]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

[propel-code-bot]

These safeguards ensure the query paths return pre-sized empty results and gracefully handle cases where the persistent index is unavailable so the merge logic continues to rely on safe defaults.

Affected Areas

• chromadb/segment/impl/vector/local_hnsw.py
• chromadb/segment/impl/vector/local_persistent_hnsw.py

This summary was automatically generated by @propel-code-bot