Normal test:
As indicated in T16074, set your profile picture by uploading an image,
then, destroy it from the command line (./bin/remove destroy MONOGRAM).
You do not see anymore a broken user image causing 404 errors,
but you see your lovely builtin image.
Corner case test:
Upload another profile picture, visit that file directly, click on {nav Attached > Detach File}
(since it was possible, so we expect somebody may click on it) and destroy that file again
from command line: it still works as intended and you see your builtin image,
and not a blank image and 404 errors in the browser requests.
Quality assurance:
Images from other users are as they were before, in all cases.