fix(falkordb): guard clone() against empty graph names

[kramerica-inc-dev]

Summary

Fixes #1650: an empty group_id crashes the FalkorDB driver with the misleading TypeError Expected a string parameter, but received <class 'str'>.

Mechanism: validate_group_id() deliberately lets '' through — the empty string is the default case for other providers — so the guard belongs in the FalkorDB driver, not in the validator. The empty string flows into FalkorDriver.clone(database=''), which builds a driver bound to an empty graph name. Because select_graph() is lazy, nothing fails at clone time; the crash only surfaces on the first query on the clone, when falkordb-py's select_graph('') raises the TypeError above (misleading, since the value is a string — reported by us as FalkorDB/falkordb-py#244).

The fix is minimal and local to falkordb_driver.py:

• clone(): a falsy database now falls back to self._database, so clone('') returns the driver itself (bound to its current database) instead of a broken clone.
• _get_graph(): the existing is None guard is widened to falsy (if not graph_name:) as defense in depth, matching the two fix locations suggested in FalkorDriver: empty group_id crashes with misleading TypeError "Expected a string parameter, but received <class 'str'>" #1650.

Production context: in our deployment this was triggered by a replayed queue entry without a group_id; the delayed crash (first query, not clone time) made it hard to trace back to the empty group_id.

Relation to #1572: complementary, not conflicting. That PR memoizes clone(); this PR normalizes the database argument before the existing branch chain. If both land, the falsy guard simply runs before (or inside) the memoized lookup.

We are the reporters of both #1650 and FalkorDB/falkordb-py#244.

Type of Change

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

Objective

N/A (bug fix).

Testing

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

New tests in tests/driver/test_falkordb_driver.py (existing mock pattern, skipped when falkordb is not installed):

• test_clone_with_empty_database_falls_back_to_current_database — clone('') returns the source driver with its database intact.
• test_clone_with_empty_database_never_selects_empty_graph — a query on a clone('') driver never calls select_graph('') (asserted on the mock client).
• test_clone_with_database_creates_new_driver — clone('group') behavior unchanged.
• test_clone_with_default_group_id_uses_default_database — the default_group_id ('_') branch stays intact.
• test_get_graph_with_empty_name_defaults_to_default_database — _get_graph('') falls back to the default database.

ruff format --check / ruff check pass; pytest tests/driver/test_falkordb_driver.py -m "not integration": 30 passed, 1 skipped. The three empty-name tests fail on unpatched main, confirming they cover the bug.

Breaking Changes

• This PR contains breaking changes

No. clone('') previously produced a driver that was guaranteed to crash on first use, so no working behavior changes.

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 #1650

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[Naseem77]

@kramerica-inc-dev Reproduced the confusing TypeError from select_graph('') on both clone('') and _get_graph(''). With your guard they fall back to the default graph and the full driver test suite passes.