[ReflectData] Provide a way for parameterised types to be nullable

Although possible with SpecificData (schema generated classes), the ReflectData currently lacks a way to have a declaration such as

java.util.List<String> myList;

to be able to generate a nullable schema

{ "type": "array", "items": ["null", "string"] }

instead of the current

{ "type": "array", "items": "string" }

There are @Union and @Nullable annotations available, but they cannot be applied to the above parameterised Collection declaration.