strawberry codegen previously choked for inputs that used the
strawberry.UNSET sentinal singleton value as a default. The intent
here is to say that if a variable is not part of the request payload,
then the UNSET default value will not be modified and the service
code can then treat an unset value differently from a default value,
etc.
For codegen, we treat the UNSET default value as a GraphQLNullValue.
The .value property is the UNSET object in this case (instead of
the usual None). In the built-in python code generator, this causes
the client to generate an object with a None default. Custom client
generators can sniff at this value and update their behavior.