texture-cache: Use image-missing image when no other icon was loaded

texture-cache: Use image-missing image when no other icon was loaded

If a given icon is not available for neither the current theme nor any
of the fallback options (default theme -"Adwaita", "gnome"...), the
call to gtk_icon_theme_lookup_by_gicon() will return a NULL value, which
returned by the Shell Toolkit as is, causing trouble in the Shell's JS
code when calling shell_app_create_icon_texture() to create an icon.

To at least mitigate the chances of this having this issue happening, we
should at least try to load the standad 'image-missing' icon from the
Icon Naming Specification spec when we receive a NULL here, so that
at least we try to show something to the user, even if it's ugly.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1121