fix(core): handle null candidate in sqlite json contains simulation

[AlessandroPomponio]

Summary

Fixes a bug in the SQLite fallback for JSON_CONTAINS where filtering by a null value would return no results instead of matching rows where the field is explicitly null or absent entirely.

High-level Changes

• ado/metastore/sql/statements.py: Added a short-circuit in simulate_json_contains_on_sqlite to handle null candidates. When the candidate is null, the function now emits a simpler json_extract(...) IS NULL query instead of delegating to json_tree-based subqueries (which only emit rows for fields that exist and would therefore never match missing or null fields).
• tests/metastore/test_sql_utils.py: Added an integration test that creates an in-memory SQLite database with three rows — one with an explicit null field, one with the field missing entirely, and one with a real value — and asserts that filtering by null returns only the first two rows.

Impact

Queries that filter metastore resources by a null metadata field now return the correct results on SQLite. No changes to the public API or database schema; the fix is internal to the SQLite JSON-contains simulation logic.

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AI Disclosure

The code and this PR description were generated using IBM Bob and reviewed manually.