fix(render): keep un-aliased NULL unwrapped so NOT survives negation

[toddyLee]

Summary

SqlalchemyRender.to_expression() wrapped every ast.Constant — including NULL — in a .label(). In WHERE operands this turned x IS NULL into a bind-param comparison, which defeats SQLAlchemy's negate optimization, so a negated null predicate compiled to the identical expression and the NOT was silently dropped:

expr = col.operate(operators.is_, null_label)   # x IS :param_1
str(expr.__invert__().compile())                # x IS :param_1  <-- NOT lost
str(col.is_(None).__invert__().compile())       # x IS NOT NULL  <-- correct without the Label

Pushed-down queries therefore returned inverted results with no error. Verified on a live deployment (ClickHouse integration):

SELECT COUNT(*) FROM db.t WHERE code='UAV' AND TypeName IS NOT NULL;   -- 1 row (correct)
SELECT COUNT(*) FROM db.t WHERE code='UAV' AND NOT (TypeName IS NULL); -- 0 rows (wrong, should be 1)
SELECT COUNT(*) FROM db.t WHERE code='UAV' AND NOT (TypeName IS NOT NULL); -- 1 row (wrong, should be 0)

Because the renderer is shared, this affects every integration that pushes SQL through SqlalchemyRender (mysql, postgres, mssql, oracle, snowflake, duckdb, bigquery, databricks, and 30+ community handlers) and is dialect-independent — reproduced rendering with MySQL, PostgreSQL and SQLite dialects.

Fix

Return sa.null() without a label when t.value is None and not t.alias. Labeled behavior is kept for aliased SELECT-list constants (SELECT NULL AS x).

Tests

5 new regression cases in tests/unit/render/test_sqlalchemyrender.py (TestNullPredicateRendering): NOT (x IS NULL), NOT x IS NULL, NOT (x IS NOT NULL), positive-form invariance, and aliased-NULL label preservation. Full file passes 20/20.

Verification

Applied the same one-line fix to a live mindsdb/mindsdb:latest deployment: all probe queries returned correct results afterwards, no regressions on the positive forms.

Fixes mindsdb/mindshub#12491

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[toddyLee]

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[todd-lee]

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[toddyLee]

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