fix for passing NSNull vs nil to Obj-C rest channel publish method

- Dart side will not pass the key if it does't have a value
- Obj-C side will query incoming _dataMap for key which will return `nil` as key will not be available

ios/Classes/AblyFlutterPlugin.m

@@ -54,15 +54,11 @@ -(nullable AblyFlutter *)ablyWithHandle:(NSNumber *)handle;
     AblyFlutterMessage *const messageData = message.message;
     NSMutableDictionary<NSString *, NSObject *>* _dataMap = messageData.message;
     NSString *channelName = (NSString*)[_dataMap objectForKey:@"channel"];
-    NSString *eventName = (NSString*)[_dataMap objectForKey:@"name"];
-    NSObject *eventData = (NSString*)[_dataMap objectForKey:@"message"];
+    NSString *const eventName = (NSString*)[_dataMap objectForKey:@"name"];
+    NSObject *const eventData = (NSString*)[_dataMap objectForKey:@"message"];
     ARTRest *client = [ably getRest:messageData.handle];
     ARTRestChannel *channel = [client.channels get:channelName];
 
-    //Handling nil cases, to be generalized based on other encounters
-    NSString *nilStr = nil;
-    if([eventName isKindOfClass:[NSNull class]]){ eventName = nilStr; }
-    if([eventData isKindOfClass:[NSNull class]]){ eventData = nilStr; }
     [channel publish:eventName data:eventData callback:^(ARTErrorInfo *_Nullable error){
         if(error){
             result([

lib/src/impl/rest/channels.dart

@@ -31,11 +31,10 @@ class RestPlatformChannel extends PlatformObject implements spec.Channel{
   @override
   Future<void> publish([String name, dynamic data]) async {
     try {
-      await this.invoke(PlatformMethod.publish, {
-        "channel": this.name,
-        "name": name,
-        "message": data
-      });
+      Map _map = { "channel": this.name, };
+      if(name!=null) _map["name"] = name;
+      if(data!=null) _map["message"] = data;
+      await this.invoke(PlatformMethod.publish, _map);
     } on PlatformException catch (pe) {
       throw spec.AblyException(pe.code, pe.message);
     }