ShortcutListBox: only add ShortcutRow when schema key is valid

[bobby285271]

This fixes a regression from c4988d9a3ed26e44fe61b3f09bb5dc071497ff4f.

The vanilla GSD does not know about the 'terminal' media-key, this result in a crash for non-ubuntu users. This restores the behavior before c4988d9, we just hide the row when the key is not valid.

https://git.launchpad.net/ubuntu/+source/gnome-settings-daemon/diff/debian/patches/ubuntu/media-keys-restore-terminal-keyboard-shortcut-schema.patch

• Closes https://github.com/elementary/switchboard-plug-keyboard/issues/426