SrcGen fast-path serialization for nullable structs (#59719)

[SkiFoD]

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Issue Details

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Author:	SkiFoD
Assignees:	-
Labels:	area-System.Text.Json, new-api-needs-documentation, community-contribution
Milestone:	-

[eiriktsarpalis]

Fixes #59719.

[layomia]

Thanks for the PR and effort here @SkiFoD. Upon further investigation, it turns out that these changes are not needed because fast-path methods for underlying nullable types are already being invoked when serializing parent nullable types. The pattern for this fast-path execution remains the same:

PersonStruct? person = new()
{
    FirstName = "Jane",
    LastName = "Doe"
};

string json = JsonSerializer.Serialize(person, someContext.NullablePersonStruct); // Fast-path for person struct is invoked.

I added tests verifying this behavior here - #61711.