[flutter_appauth] iOS app would crash when authorizationCode was nil

[kennethj]

iOS app would crash when authorizationCode was nil

Error message in Obj-C:

The operation couldn’t be completed. *** -[__NSDictionaryM setObject:forKey:]: object cannot be nil (key: authorizationCode)

The exception is thrown here: https://github.com/MaikuB/flutter_appauth/blob/master/flutter_appauth/ios/Classes/AppAuthIOSAuthorization.m#L86-L89

This PR #588 prevents the app from crashing by catching the exception and sending it to the UI.