fix(firebase_messaging): fix null apple notification when sound is of type String (#17770)

Author: abdalin

packages/firebase_messaging/firebase_messaging/ios/firebase_messaging/Sources/firebase_messaging/FLTFirebaseMessagingPlugin.m

@@ -992,7 +992,7 @@ + (NSDictionary *)remoteMessageUserInfoToDict:(NSDictionary *)userInfo {
     if (apsDict[@"sound"] != nil) {
       if ([apsDict[@"sound"] isKindOfClass:[NSString class]]) {
         // message.notification.apple.sound
-        notification[@"sound"] = @{
+        notificationIOS[@"sound"] = @{
           @"name" : apsDict[@"sound"],
           @"critical" : @NO,
           @"volume" : @1,