xwayland: don't reject present request on unmapped windows

This match the behavior of the modesetting server.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
1 job for fix-present-xwayland in 1 minute and 10 seconds (queued for 3 minutes and 39 seconds)
latest
Status Name Job ID Coverage
  Test
passed job #41994

00:01:10