Make FlutterEngine accept nil in setMessageHandlerOnChannel:binaryMessageHandler: when deallocating. #57151
Description
Related issue: #56996
On iOS, Plugins may try to set their message handlers to nil when they become detached. They don't have the context to know if it is detaching from a live engine or a dying engine. Inside of FlutterEngine we assert that people only call that method for live engines, we should ignore it if the engine is dying.