code: public Class<?> resolveReturnTypeGeneric() {
          final List<Class<?>> res = resolveReturnTypeGenerics();
          return res.isEmpty() ? null : res.get(0);
      }
docs: Shortcut for {@link #resolveReturnTypeGenerics()} useful for single generic types or
  when just first generic required.
  
  @return first resolved generic or null if type doesn't use generics