fix: support untyped NULL input for median

[Sigma-Ma]

Which issue does this PR close?

• Closes Bug + Regression: median(null) fails in v54 #24077.

Rationale for this change

median(NULL) worked in DataFusion 53 but regressed in DataFusion 54. The untyped NULL argument remained DataType::Null, causing physical planning to create a median accumulator for an unsupported input type.

What changes are included in this PR?

• Use explicit type coercion for median.
• Coerce untyped NULL and integer inputs to Float64.
• Preserve Float and Decimal input types.
• Add a SQL logic regression test for median(NULL).

Are these changes tested?

Yes.

• cargo fmt --all -- --check
• cargo test --profile=ci --test sqllogictests -- aggregate.slt
• cargo test -p datafusion-functions-aggregate
• cargo clippy -p datafusion-functions-aggregate --all-targets --all-features -- -D warnings

Are there any user-facing changes?

Yes. median(NULL) now returns a typed Float64 NULL instead of failing during physical execution.

[Sigma-Ma]

Hi, this is my first contribution to DataFusion. Could a committer please trigger CI when convenient? Thanks!