fix: handle Null type in try_merge for Struct, List, LargeList, and Union

[zhuqi-lucas]

Which issue does this PR close?

Field::try_merge correctly handles DataType::Null for primitive types
and when self is Null, but fails when self is a compound type (Struct,
List, LargeList, Union) and from is Null. This causes Schema::try_merge
to error when merging schemas where one has a Null field and another has a
concrete compound type for the same field.

This is common in JSON inference where some files have null values for
fields that are structs/lists in other files.

• Closes #9523

Rationale for this change

Add DataType::Null arms to the Struct, List, LargeList, and Union
branches in Field::try_merge, consistent with how primitive types
already handle it.

What changes are included in this PR?

Add DataType::Null arms to the Struct, List, LargeList, and Union
branches in Field::try_merge, consistent with how primitive types
already handle it.

Are these changes tested?

• Added test test_merge_compound_with_null covering Struct, List,
  LargeList, and Union merging with Null in both directions.
• Existing tests continue to pass.

Are there any user-facing changes?

No

[Copilot]

Pull request overview

Fixes Field::try_merge to treat DataType::Null as compatible when merging into compound field types (Struct, Union, List, LargeList), preventing schema-merge failures common during JSON schema inference.

Changes:

• Add DataType::Null match arms for Struct/Union/List/LargeList in Field::try_merge to mark the field nullable instead of erroring
• Add a regression test covering merges of compound types with Null

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[zhuqi-lucas]

cc @alamb , i met some corner cases when using infer schema, and i found it's the bug from arrow side. I hope this PR is helpful, thanks!

error:

Error: Arrow error: Schema error: Fail to merge schema field 'item' because the from data_type = Null is not DataType::Struct