Fix empty list filter raising IndexError instead of WeaviateInvalidInputError

[metawake]

Summary

• Passing an empty list to filter methods (.equal([]), .not_equal([]), .less_than([]), .less_or_equal([]), .greater_than([]), .greater_or_equal([])) caused an IndexError: list index out of range because the code accessed value[0] without checking if the list was empty.
• Added inline empty-list validation to raise a descriptive WeaviateInvalidInputError in each affected method, matching the existing pattern used by contains_any/contains_all/contains_none.
• Added defense-in-depth guards in the gRPC and REST converter methods (_FilterToGRPC, _FilterToREST) to prevent IndexError if an empty list reaches the serialization layer.

Closes #933

Files changed

• weaviate/collections/classes/filters.py — inline empty-list checks in equal, not_equal, less_than, less_or_equal, greater_than, greater_or_equal
• weaviate/collections/filters.py — guards in _FilterToGRPC list methods and _FilterToREST.__parse_filter
• test/collection/test_filter.py — 6 new unit tests covering each affected filter method

Design note

The empty-list validation is intentionally inlined in each method rather than extracted into a shared helper, to stay consistent with how contains_any/contains_all/contains_none already handle validation in this codebase.

Test plan

• pytest test/collection/test_filter.py -v — all 30 tests pass (including 6 new)
• No Docker/Weaviate instance needed — pure client-side validation

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

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[metawake]

I agree with the CLA!