fix: UNIQUE constraint with NULLs incorrectly collapses GROUP BY groups

[xiedeyantu]

Which issue does this PR close?

• Closes Incorrect query results for GROUP BY with UNIQUE constraint #21507

Rationale for this change

UNIQUE constraints can contain multiple NULL values, so they do not guarantee row-level uniqueness in SQL semantics. The optimizer was incorrectly treating nullable unique constraints as functional dependencies that could reduce GROUP BY keys, which collapsed distinct NULL rows into a single group.

What changes are included in this PR?

This PR updates functional-dependency handling so nullable dependencies derived from UNIQUE constraints are not used to eliminate GROUP BY expressions. It also adds a regression test covering the NULL case from the issue report.

Are these changes tested?

Yes. I ran:

• cargo fmt --all
• cargo clippy -p datafusion-common --all-targets -- -D warnings
• cargo test -p datafusion-common functional_dependencies
• cargo test -p datafusion-sqllogictest --test sqllogictests -- group_by

Are there any user-facing changes?

Yes. Queries that group by nullable UNIQUE columns will no longer return incorrect aggregated results when multiple NULL values are present.