The title field in the push notification should be nullable

[ttypic]

If push notification come without title field SDK throws exception:

The following _TypeError was thrown during a platform message callback:
type 'Null' is not a subtype of type 'String' in type cast

When the exception was thrown, this was the stack:
#0      new Notification.fromMap
(package:ably_flutter/src/push_notifications/src/notification.dart:29:42)
#1      new RemoteMessage.fromMap
(package:ably_flutter/src/push_notifications/src/remote_message.dart:37:28)
#2      Codec._decodeRemoteMessage
...

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