message: fix: allow setting attributes to null
  fixes
diff: @@ -371,7 +371,10 @@ export class JSONAPISerializer
  }
  const serializer = this._serializers[attrOptions.type];
  if (serializer) {
  - value = serializer.serialize(value, attrOptions.serializationOptions);
  + value =
  + value === null
  + ? null
  + : serializer.serialize(value, attrOptions.serializationOptions);
  }
  deepSet(
  resource,
  