Stop defaulting missing images to deprecated stock icons

GTK_STOCK_MISSING_IMAGE/"gtk-missing-image" has been deprecated for a
while now and Adwaita does not ship it anymore under that name. Better
prefer the new "image-missing" replacement.
3 jobs for master in 9 minutes and 18 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #244861
flatpak

00:08:49

 
  Review
passed #244862
review

00:00:28

manual #244863
allowed to fail manual
stop_review