pkglist: Canonicalize "no packages" to "no database found"

It can happen that we successfully query zero packages.  For example,
when rpm-ostree on RHEL8 (assuming bdb database) is trying to parse an
ostree commit generated from fedora (sqlite rpmdb), librpm will just give us
nothing.  Eventually perhaps we may need to fall back to actually running
the target commit as a container just to get this data for cases like that.

Specifically, I hit this doing
`rpm-ostree rebase --experimental ostree-unverified-image:registry:quay.io/cgwalters/fcos-derivation-example`
on RHCOS8.

src/app/rpmostree-builtin-shlib-backend.cxx

@@ -76,6 +76,13 @@ impl_packagelist_from_commit (OstreeRepo *repo, const char *commit, GError **err
     }
 
   g_autoptr(GVariant) pkgs = rpmostree_variant_pkgs_from_sack (rsack);
+  // It can happen that we successfully query zero packages.  For example,
+  // when rpm-ostree on RHEL8 (assuming bdb database) is trying to parse an
+  // ostree commit generated from fedora (sqlite rpmdb), librpm will just give us
+  // nothing.  Eventually perhaps we may need to fall back to actually running
+  // the target commit as a container just to get this data for cases like that.
+  if (g_variant_n_children (pkgs) == 0)
+    return g_variant_new_maybe ((GVariantType*) RPMOSTREE_SHLIB_IPC_PKGLIST, NULL);
   return g_variant_ref_sink (g_variant_new_maybe ((GVariantType*) RPMOSTREE_SHLIB_IPC_PKGLIST, pkgs));
 }