nspawn: copy BindUser= setting from settings only if set

Let's only pick this up from the settings if actually set.

As in the previous commit this makes sure that an empty settings file in
--settings=override mode is really a NOP.

src/nspawn/nspawn.c

@@ -4452,7 +4452,8 @@ static int merge_settings(Settings *settings, const char *path) {
                 }
         }
 
-        if ((arg_settings_mask & SETTING_BIND_USER) == 0)
+        if ((arg_settings_mask & SETTING_BIND_USER) == 0 &&
+            !strv_isempty(settings->bind_user))
                 strv_free_and_replace(arg_bind_user, settings->bind_user);
 
         if ((arg_settings_mask & SETTING_NOTIFY_READY) == 0 &&