isScalarType for non-null types

[tomas-c]

TLDR:

isScalarType should support NotNullType.

Without it, mapper.convertValue(value, typeReference) is always called and sometimes fails with latest Jackson.

Background:

After upgrading from Spring Boot 2.1.8 to 2.2.2 I started getting an exception when using ApolloScalars.Upload scalar type. The exception said:

No serializer found for class java.io.FileDescriptor and no properties discovered to create BeanSerializer

Turns our new version of Spring came with a new version of Jackson which had removed conversion short-circuiting

[rrva]

#342 fixes this