Prevent empty grouping sets from being eliminated on empty input

[xiedeyantu]

Which issue does this PR close?

• Closes GROUPING SETS (()) returns no rows on empty input #22040

Rationale for this change

GROUPING SETS (()) should return one row even when the input is empty, because the empty grouping set represents a global aggregate. In the current behavior, queries such as WHERE false GROUP BY GROUPING SETS(()) can incorrectly be optimized into an empty result, returning 0 rows instead of the expected 1 row. This change fixes that SQL semantics bug.

What changes are included in this PR?

This PR makes two related fixes:

1. It updates the empty-relation propagation rule so aggregates with an empty grouping set are not eliminated.
2. It updates the grouped hash aggregate execution path so empty grouping sets are initialized correctly and still produce output on empty input.

It also adds regression coverage in grouping.slt for:

• GROUPING SETS (()) on empty input
• COUNT(*) on empty input
• mixed grouping sets where only the empty grouping set should produce a row

Are these changes tested?

Yes. I ran:

• cargo test -p datafusion-sqllogictest -- grouping
• cargo test -p datafusion-optimizer -- propagate_empty
• cargo clippy --tests --all-features -p datafusion-physical-plan -p datafusion-optimizer -- -D warnings
• cargo fmt --check

Are there any user-facing changes?

Yes. Queries using GROUPING SETS (()), and other grouping-set forms that include the empty set such as ROLLUP and CUBE, will now return the correct row on empty input instead of incorrectly returning no rows.