display: Add guards for NULL display configuration

This may happen under some conditions. Possibly due to a race condition
(i.e. we did not receive any configuration from mutter yet) or also if
we are not running on GNOME.

Add guards for NULL configuration. This configuration is never
applicable and mostly clears the UI.

Fixes: #604
11 jobs for benzea/display-more-error-handling in 3 minutes and 34 seconds (queued for 2 seconds)
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