Expression simplification of `col ~ '.*'` to `col IS NOT NULL` drops NULL semantics

[viirya]

Describe the bug

Expression simplification rewrites col ~ '.*' to col IS NOT NULL, which loses SQL NULL semantics. For a NULL input, NULL ~ '.*' is NULL (three-valued logic), but NULL IS NOT NULL is false. So in a projection context the rewrite returns false where the correct answer is NULL.

The !~ (RegexNotMatch) branch of the same rule is already NULL-aware (col IS NULL AND NULL); only the ~ (RegexMatch) branch drops the NULL. Same class as #24246, but a different rule (regex .*).

To Reproduce

SELECT s, s ~ '.*' AS m
FROM (VALUES (CAST(NULL AS VARCHAR)), ('x')) t(s);

Actual:

+---+-------+
| s | m     |
+---+-------+
|   | false |   <- wrong, should be NULL
| x | true  |
+---+-------+

Expected (what NULL semantics require):

+---+------+
| s | m    |
+---+------+
|   | NULL |
| x | true |
+---+------+

Expected behavior

col ~ '.*' is true for a non-NULL string and NULL for a NULL input, matching NestedLoopJoin-free evaluation.

Additional context

Root cause: simplify_regex_expr in datafusion/optimizer/src/simplify_expressions/regex.rs rewrites the ~ '.*' case to left.is_not_null(). In a WHERE filter this is fine (both FALSE and NULL reject the row, which is why existing SLT filter tests didn't catch it), but it is wrong in a general/projection context. Fix + regression test coming.