Annotations Polymorphism does not work with default S.T.J. discriminator name

[pinkfloydx33]

The JsonPolymorphicAttribute.TypeDiscriminatorPropertyName is nullable. When it's not provided, System.Text.Json defaults to "$type" per the documentation:

Gets or sets a custom type discriminator property name for the polymorhic type. Uses the default '$type' property name if left unset.

When using EnableAnnotations, the built-in AnnotationsDiscriminatorNameSelector method will return a null value if it finds a JsonPolymorphicAttribute where the user hasn't explicitly specified the value (hoping to leverage the defaults). This in turn causes the polymorphism/discriminator/etc. to be completely absent from the generated specification.

Swashbuckle.AspNetCore/src/Swashbuckle.AspNetCore.Annotations/AnnotationsSwaggerGenOptionsExtensions.cs

Lines 115 to 125 in 0c61531

	#if NET7_0_OR_GREATER
	var jsonPolymorphicAttributes = baseType.GetCustomAttributes(false)
	.OfType<JsonPolymorphicAttribute>()
	.FirstOrDefault();
	if (jsonPolymorphicAttributes != null)
	{
	return jsonPolymorphicAttributes.TypeDiscriminatorPropertyName;
	}
	#endif

I believe that the correct behavior would be to coalesce to "$type" IIF the JsonPolymorphicAttribute exists:

             if (jsonPolymorphicAttributes != null) 
             { 
                 return jsonPolymorphicAttributes.TypeDiscriminatorPropertyName ?? "$type";
             }