wayland/keyboard: Guard against null resource

Florian Müllner requested to merge fmuellner/mutter:shutdown-crash into main

This fixes the following abort (during shutdown with headless backend):

Thread 1 "gnome-shell" received signal SIGTRAP, Trace/breakpoint trap. 0x00007ffff7946ff1 in g_logv () from /lib64/libglib-2.0.so.0

We started hitting that abort in gnome-shell's closeWithActiveWindows test, after the test window was ported to GTK4. It's possible that there's a GTK bug in play, but we shouldn't crash regardless.

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