Update RuntimeException message when no data has been found

[mikemeijer]

I discovered an inconsistency in the getData method of the Illuminate\Notifications\Channels\BroadcastChannel.
When both the toBroadcast and toArray doesn't exists it only shows the Notification is missing toArray method. while it shows Notification is missing toDatabase / toArray method. in the getData of the Illuminate\Notifications\Channels\DatabaseChannel.

I've update this line to be more consistent in the exception message of both classes.