Fix crash when deleting device in OmemoKeysView

[matthewrfennell]

In this commit, we create a fallback/default value for the fingerprint when no data is retrieved from the database.

This is needed to prevent crashes when removing a device from the list.

The order of operations without this fallback is:

• User clicks on the delete button to remove a device
• The device gets removed from the DB
• A notification is sent to the view
• The view picks up the notification, via the ObservableObject
• SwiftUI decides to recreate the view, while playing the animation
• self.fingerprint is now empty, as there is no such device in the DB
• We call signalHexKeyWithSpaces with this empty fingerprint
• This function assumes the result is more than 2 characters long
• This causes the crash