Fix system notice on login_page.php

When $g_display_errors[E_ALL] and/or $g_display_errors[E_USER_ERROR] is
not set, the login page throws a system notice when checking for
E_USER_ERROR setting.

Test condition has been modified to include isset() check to avoid that.

Follow up on 0c138467cb6f88ca1c40275c00c9045f421b7b50