libpriv/core: allow NULL for treespec

Let callers pass `NULL` for the `@spec` argument. This essentially means
that they're not interested in actually doing any installation/commit
operations, but just the semantics that the core uses when setting up
libdnf wrt e.g. install and source root and enabled repos = required,
etc...

Prep for `makecache`.

src/libpriv/rpmostree-core.c

@@ -545,7 +545,14 @@ rpmostree_context_setup (RpmOstreeContext    *self,
   /* This exists (as a canonically empty dir) at least on RHEL7+ */
   static const char emptydir_path[] = "/usr/share/empty";
 
-  self->spec = g_object_ref (spec);
+  /* allow NULL for treespec, but canonicalize to an empty keyfile for cleaner queries */
+  if (!spec)
+    {
+      g_autoptr(GKeyFile) kf = g_key_file_new ();
+      self->spec = rpmostree_treespec_new_from_keyfile (kf, NULL);
+    }
+  else
+    self->spec = g_object_ref (spec);
 
   g_variant_dict_lookup (self->spec->dict, "releasever", "&s", &releasever);