vconsole-setup: don't fail with an empty keymap

The new meson option 'default-keymap' means that by default there's always
a keymap to configure, so vconsole-setup fails. Allow to set
-Ddefault-keymap="" to disable it and skip gracefully again.

Follow-up for f129d0e

src/vconsole/vconsole-setup.c

@@ -280,7 +280,7 @@ static int keyboard_load_and_wait(const char *vc, Context *c, bool utf8) {
         map_toggle = context_get_config(c, VC_KEYMAP_TOGGLE);
 
         /* An empty map means kernel map */
-        if (!map)
+        if (isempty(map))
                 return 0;
 
         args[i++] = KBD_LOADKEYS;