message: Fix incorrectly handled undefined case in write
diff: @@ -13,12 +13,12 @@ const writeResolver: FieldResolver = (
  const fieldValue = rootValue[info.resultKey || fieldName];
  const parentKey = store.keyOfEntity(rootValue);
  - if (parentKey === null) {
  + if (parentKey === null || fieldValue === undefined) {
  return null;
  } else if (parentKey === 'Mutation' || parentKey === 'Subscription') {
  // We do traverse but we don't store Mutation & Subscription themselves
  return fieldValue;
  - } else if (fieldValue === null || fieldValue === undefined) {
  + } else if (fieldValue === null) {
  // Clear stored field since value is null
  store.writeEntityValue(parentKey, fieldName, null);
  return null;
  