gutils: assert against calling g_set_application_name()

g_set_application_name() guards against being reset, but it doesn't
remember whether it was set, it only checks whether g_application_name
was set to non-NULL. When allowing g_set_application_name(NULL) that leads
to odd behaviors, like:

    g_set_application_name(NULL);
    g_set_application_name("foo");

would not warn.

Disallow that and assert against a NULL application_name.

Note that application_name argument is also not marked as "(nullable)".

glib/gutils.c

@@ -1233,6 +1233,8 @@ g_set_application_name (const gchar *application_name)
 {
   gboolean already_set = FALSE;
 
+  g_return_if_fail (application_name);
+
   G_LOCK (g_application_name);
   if (g_application_name)
     already_set = TRUE;