Fix whitespace-only LanceDB db_uri fallback

[yuzhiyang1]

Description

Fixes LanceDB vector store db_uri validation so whitespace-only values fall back to the default db_uri instead of resolving to the current working directory.

Related Issues

Fixes #2381

Proposed Changes

• Call strip() when checking whether the LanceDB db_uri is blank.
• Add a unit test covering whitespace-only db_uri values.
• Add a semversioner patch change entry.

Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).

Additional Notes

Validation run locally:

• uv run pytest tests/unit/config/test_config.py -q
• uv run ruff format packages/graphrag/graphrag/config/models/graph_rag_config.py tests/unit/config/test_config.py --check
• uv run ruff check packages/graphrag/graphrag/config/models/graph_rag_config.py tests/unit/config/test_config.py

[yuzhiyang1]

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