gio: Mark g_task_get_source_object as nullable
gio: Mark g_task_get_source_object as nullable

In conjunction with ca4fe5942afa5d24f2c99f12d41598cf04b7718f

Reviewed-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib