Fix JAXB collections deserializing null

[rob-bygrave]

The JAXB collections don't have setters but instead lazy initialise collections on getter methods, the generated code uses .addAll() which doesn't support the case when NULL is returned (vs empty collection).

Fix is to add a nullToEmpty() helper method for this case (JAXB style collections).

Fix to generated code adds the Types.nullToEmpty() like:

case "tags": 
  _$myJaxbType.getTags().addAll(Types.nullToEmpty(listStringJsonAdapter.fromJson(reader)));
  break;