Use underlying type converter for nullable type (#84208)

Use underlying type converter for nullable type (#84208)

* Use underlying type converter for nullable type

* verify that we use underlying type converter for nullable types

* rollback a little change

* Update src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/JsonSourceGeneratorTests.cs

* Update src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/JsonSourceGeneratorTests.cs

* Use consistent indentation.

* Fix runtime support for nullable properties with custom converters.

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Co-authored-by: Eirik Tsarpalis <eirik.tsarpalis@gmail.com>