Keep physical scalar subqueries nullable

[osipovartem]

Summary

• mark the physical expression for an uncorrelated scalar subquery nullable
• add a planner-level test whose inner projection is non-nullable

Why

The logical expression contract was corrected in #64, but physical planning independently copied nullability from the subquery's projected field. An empty scalar subquery returns NULL even when that projected field is declared non-nullable, so the physical expression must preserve the SQL-level nullable contract.

This was found by the Rustice live-Snowflake oracle for:

SELECT (SELECT column1 FROM VALUES (2) WHERE FALSE) AS scalar_empty;

Validation

• cargo +1.97.0 test -p datafusion-physical-expr -q (1588 passed, 2 ignored; all 13 secondary tests passed)
• cargo +1.97.0 clippy -p datafusion-physical-expr --all-targets -- -D warnings
• cargo +1.97.0 fmt --all -- --check