[ShellAppSystem] Don't crash if the menu system is empty

This is a bit pathological, but if your menu tree is empty, we
shouldn't crash.

src/shell-app-system.c

@@ -309,10 +309,16 @@ reread_entries (ShellAppSystem     *self,
   g_slist_free (*cache);
   *cache = NULL;
 
+  if (!trunk)
+    {
+      *cache = NULL;
+    }
+  else
+    {
       *cache = gather_entries_recurse (self, *cache, unique, trunk);
-
       gmenu_tree_item_unref (trunk);
     }
+}
 
 static void
 cache_by_id (ShellAppSystem *self, GSList *apps)