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?

• Keep the existing declarative signature for median.
• Handle DataType::Null in physical execution using NoopAccumulator.
• Use a Null state field for Null input.
• Fall back to the generic groups accumulator adapter for Null input.
• 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 NULL with DataType::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!

[Jefffrey]

it would be preferable to keep the old signature if possible, as we prefer avoiding user defined unless strictly necessary

could we fix the physical execution path to account for null type input?

[niebayes]

agree with Jefffrey

[Sigma-Ma]

Thanks for the suggestion. I restored the original declarative signature and moved the Null handling to the physical execution path.

Null input now uses the existing NoopAccumulator pattern, has a matching Null state field, and falls back to the generic groups accumulator adapter. I also updated the regression test to expect NULL with DataType::Null.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.02%. Comparing base (aa38d3c) to head (28c4453).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24104      +/-   ##
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- Coverage   81.03%   81.02%   -0.01%     
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  Files        1105     1105              
  Lines      379846   379859      +13     
  Branches   379846   379859      +13     
==========================================
- Hits       307793   307790       -3     
- Misses      53831    53841      +10     
- Partials    18222    18228       +6     

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