message: fix(usercontext): Add null check for user properties
  This adds additional checks for user parameters
  Fixes
diff: @@ -391,9 +391,15 @@ RCT_EXPORT_METHOD(captureEvent:(NSDictionary * _Nonnull)event)
  userId = [NSString stringWithFormat:@"%@", user[@"id"]];
  }
  SentryUser *sentryUser = [[SentryUser alloc] init];
  + if (nil != userId) {
  sentryUser.userId = userId;
  + }
  + if (nil != user[@"email"]) {
  sentryUser.email = [NSString stringWithFormat:@"%@", user[@"email"]];
  + }
  + if (nil != user[@"username"]) {
  sentryUser.username = [NSString stringWithFormat:@"%@", user[@"username"]];
  + }
  sentryUser.extra = [RCTConvert NSDictionary:user[@"extra"]];
  return sentryUser;
  }
  