fix: handle list-level NULLs in NOT filters

[fenfeng9]

closes #5904

LabelListIndex drops list-level NULL rows during unnest, so NOT filters can return NULL lists incorrectly.

Changed:

• Persist list-level NULL rows in label_list_nulls.lance and merge into null sets.
• Treat missing nulls file as empty for backward compatibility.
• Warn when using an old/possibly incomplete LabelList index on nullable columns.

[codecov]

Codecov Report

❌ Patch coverage is 84.75610% with 25 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/label_list.rs	82.17%	8 Missing and 10 partials ⚠️
rust/lance/src/index/scalar.rs	53.84%	4 Missing and 2 partials ⚠️
rust/lance-index/src/scalar/lance_format.rs	98.00%	0 Missing and 1 partial ⚠️

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