Replace isset($_POST) with !empty($_POST)

The former is always true (outside of post method it is an empty array
and not null, and it cannot be `unset()`).

The check was as such either meaningless or was supposed to check that
something was sent. This commit assume the later.

inc/auth.php

@@ -12,7 +12,7 @@
  * @version 1.0
  */
 
-if ($_SERVER['REQUEST_METHOD'] == 'POST' && isset($_POST)) {
+if ($_SERVER['REQUEST_METHOD'] == 'POST' && !empty($_POST)) {
     $input_username = $_POST['login'];
     $input_password = $_POST['passw'];