🐛 fix(TypeAdapter.cs): Make source parameter nullable

[msadeqsirjani]

Changes

• The source parameter in the Adapt method is now nullable, which allows for null values to be passed in without throwing a null reference exception. This improves the robustness of the code and makes it more defensive against unexpected null values.

@eswann @chaowlert @chaowlert @satano

[andrerav]

Thanks!