[ml-natural-language][ios] userID is now required on the iOS SDK - use a default value if non provided (android is still optional)

packages/ml-natural-language/ios/RNFBMlNaturalLanguage/RCTConvert+FIRTextMessageArray.m

@@ -25,7 +25,7 @@
       [FIRTextMessage alloc]
       initWithText:messageDict[@"text"]
          timestamp:[[messageDict valueForKey:@"timestamp"] doubleValue]
-            userID:messageDict[@"remoteUserId"] ? messageDict[@"remoteUserId"] : nil
+            userID:messageDict[@"remoteUserId"] ? messageDict[@"remoteUserId"] : @"__rnfb_local_user"
        isLocalUser:messageDict[@"remoteUserId"] ? NO : YES
   ];
   return firTextMessage;