Backport !1992 “gs-plugin-appstream: Fix a crash when no appstream data is found” to gnome-46
The xb_silo_query() can return NULL, thus count with it, instead of dereferencing a NULL pointer.
Reported downstream: https://bugzilla.redhat.com/show_bug.cgi?id=2272028
Trivial backport of !1992 (merged) to gnome-46.