build: Fix some harmless compiler warnings

Some (newer?) GCC versions complain when a g_auto variable isn't
initialized when declared, even when the initialization is guaranteed
to happen before the variable is used or goes out of scope.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2298

subprojects/extensions-tool/src/command-create.c

@@ -89,7 +89,7 @@ create_metadata (GFile       *target_dir,
 static gboolean
 copy_extension_template (GFile *target_dir, GError **error)
 {
-  g_auto (GStrv) templates;
+  g_auto (GStrv) templates = NULL;
   char **s;
 
   templates = g_resources_enumerate_children (TEMPLATE_PATH, 0, NULL);