gio: g_menu_item_set_icon should not fail if icon is NULL
gio: g_menu_item_set_icon should not fail if icon is NULL

It allows passing a NULL icon to unset the icon and thus should not
log a critical warning if used like that.

https://bugzilla.gnome.org/show_bug.cgi?id=753285

tchakabam/glib,endlessm/glib,MathieuDuponchelle/glib,Distrotech/glib,ieei/glib,endlessm/glib,ieei/glib,MathieuDuponchelle/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,gale320/glib,Distrotech/glib,gale320/glib,mzabaluev/glib,mzabaluev/glib,tchakabam/glib,ieei/glib,MathieuDuponchelle/glib,endlessm/glib,cention-sany/glib,gale320/glib,Distrotech/glib,johne53/MB3Glib,johne53/MB3Glib,ieei/glib,mzabaluev/glib,cention-sany/glib,johne53/MB3Glib,tchakabam/glib,Distrotech/glib,MathieuDuponchelle/glib,tchakabam/glib,tchakabam/glib,Distrotech/glib,gale320/glib,mzabaluev/glib,johne53/MB3Glib,johne53/MB3Glib,cention-sany/glib,cention-sany/glib,gale320/glib,MathieuDuponchelle/glib,mzabaluev/glib,cention-sany/glib,ieei/glib