fix: guard empty FalkorDB fulltext queries

[FUY25]

Summary

Fixes an issue in FalkorDriver.build_fulltext_query() where a stopword-only query could produce invalid RediSearch
syntax such as (@group_id:"standups") (). This PR returns an empty string when the filtered query is empty and adds
a regression test covering that case.

Type of Change

• Bug fix
• New feature
• Performance improvement
• Documentation/Tests

Objective

This is a bug fix, so no new feature or performance objective applies.

Testing

• Unit tests added/updated
• Integration tests added/updated
• All existing tests pass

Locally verified with:

• uv run pytest tests/utils/search/test_search_security.py -k stopword_only -v
• uv run pytest tests/utils/search/test_search_security.py -v
• uv run ruff check graphiti_core/driver/falkordb_driver.py tests/utils/search/test_search_security.py

Breaking Changes

• This PR contains breaking changes

If this is a breaking change, describe:

• Not applicable

Checklist

• Code follows project style guidelines (make lint passes)
• Self-review completed
• Documentation updated where necessary
• No secrets or sensitive information committed

Related Issues

Closes #1337

[danielchalef]

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