We use a GtkIconTheme (thus icon-theme, thus xsettings, thus x11) just to
grab a "missing icon" icon to show in place. Relax this requirement that
surfaces for icon/mini-icon will be set, and just let it have NULL here.
It seems better to have the callers (presumably UI layers) aware of this
and set a proper icon by themselves, but AFAICS there is none in sight,
not even plain mutter seems to use MetaWindow::[mini-]icon. Probably
worth a future cleanup.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/420