restorePreviousSignInWithCompletion() call crash if the completion is null #289

- (void)restorePreviousSignInWithCompletion:(nullable void (^)(GIDGoogleUser *_Nullable user,
                                                               NSError *_Nullable error))completion {
  [self signInWithOptions:[GIDSignInInternalOptions silentOptionsWithCompletion:
                           ^(GIDSignInResult *signInResult, NSError *error) {
    if (signInResult) {
      completion(signInResult.user, nil);
    } else {
      completion(nil, error);
    }
  }]];
}