fix: handle NULL elements in LABEL_LIST index results and explain_plan

[fenfeng9]

closes #5682

changes:

• Make bitmap search drop NULL rows that overlap TRUE rows to fix LABEL_LIST results with NULL items.
• Allow nullable list literals in LabelListQuery::to_expr to prevent explain_plan() panics.
• Add Python tests covering NULL list elements and explain behavior.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar.rs	50.00%	1 Missing ⚠️

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[fenfeng9]

PTAL @westonpace .