Fix JsonSerializer throws when serializing null value in a value-type nullable union case

[DeagleGross]

I've added roundtrip test on existing union ValueTypeNullablePairUnion(int, int?); to verify that nullable case value works well for valueType. There are existing tests checking the same for string?.

Fixes #128688

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[Copilot]

Pull request overview

Fixes #128688 where serializing a union with a value-type nullable case (e.g., union ValueTypeNullablePairUnion(int, int?)) holding a null value would throw, because the deconstructor returns (typeof(int), null) and dispatching null to the int converter fails in UnboxOnWrite<T>. The write path now short-circuits to JSON null when the case type is a value type and the case value is null, mirroring the existing null-handling on the read path.

Changes:

• In JsonUnionConverter<TUnion>.OnTryWrite, write JSON null directly when the deconstructor returns a value-type case with a null value.
• Add UnionWithValueTypeNullableCase_SerializesNullAsJsonNull and UnionWithValueTypeNullableCase_RoundTripsNullValue tests over the existing ValueTypeNullablePairUnion, exercised via both reflection and source-generation contexts.

Show a summary per file

File	Description
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Union/JsonUnionConverter.cs	Adds the null-value/value-type guard before dispatching to the case converter.
src/libraries/System.Text.Json/tests/Common/UnionTests.cs	Adds serialization and roundtrip tests for value-type nullable union cases.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1

[eiriktsarpalis]

The failure suggests to me that caseTypeInfo.Converter is JsonConverter<int> rather than JsonConverter<int?> which should handle null values without issue. So I suspect this might be an issue with either the union metadata or the deconstructor returning an incorrect case type.

Are you hitting this using the source generator, reflection, or both?

[DeagleGross]

I got it in both source gen and reflection. Not sure why it hits this specific converter though. I can take a look later