clutter-device-evdev: Get devices from main seat if no real seat is set

clutter-device-evdev: Get devices from main seat if no real seat is set

In devices such as ARM boards there could be no input devices connected on
startup, leading to a crash when we try to process artificial events that
could be queued (as gnome-shell does when syncing pointer).

Those events still should refer to a device and, in case we don't have one
provided by libinput we should still return the core devices defined in the
main seat.

(cherry picked from commit 3e85ac8131fc76312a2382d83df6926b88a37415)

Origin: upstream, 3.28.3, commit:2a6782dc10147a5a45f90ac318ba440f21f819d6

Gbp-Pq: Name clutter-device-evdev-Get-devices-from-main-seat-if-no-rea.patch