Fix serialization of non-nullable enums using _$xxxEnumMap

Before this change, serializing non-nullable enums in a subtype was incorrectly producing nullable string types.

For example, serializing a class member of type Map<enum, String> produced a type Map<String?, dynamic> where Map<String, dynamic> should have been produced.

Nullable enums should still produce nullable types, eg. serializing List<enum?> should produce List<String?>

fixes 1145

json_serializable/CHANGELOG.md

@@ -4,6 +4,8 @@
   of type `MyClass?`. ([#822](https://github.com/google/json_serializable.dart/issues/822))
 - Added support for `JsonSerializable(converters: <JsonConverter>[])`
   ([#1072](https://github.com/google/json_serializable.dart/issues/1072))
+- Fix issue with serialization of non-nullable enumerations emitting a nullable
+  type ([#1146](https://github.com/google/json_serializable.dart/pull/1146))
 
 ## 6.2.0