- Fix the delete of blacklist/whitelist records under NGINX + PHP-FPM

- Fix multiple php warning/error messages when this scripts are executed from AJAX requests

Example errors/warnings:

2019/01/15 13:22:22 [error] 1408#1408: *2535 FastCGI sent in stderr: "PHP message: PHP Notice:  Undefined variable: api in /var/www/html/admin/scripts/pi-hole/php/sub.php on line 16
PHP message: PHP Warning:  Cannot modify header information - headers already sent by (output started at /var/www/html/admin/scripts/pi-hole/php/sub.php:8) in /var/www/html/admin/scripts/pi-hole/php/auth.php on line 81
PHP message: PHP Warning:  session_start(): Cannot start session when headers already sent in /var/www/html/admin/scripts/pi-hole/php/auth.php on line 93

Signed-off-by: Michael Epstein <mepstein@mediabox.cl>

scripts/pi-hole/php/add.php

@@ -5,13 +5,14 @@
 *
 *  This file is copyright under the latest version of the EUPL.
 *  Please see LICENSE file for your rights under this license. */
+
 require_once('auth.php');
 
 $type = $_POST['list'];
 
 // Perform all of the verification for list editing
 // when NOT invoked and authenticated from API
-if (!$api) {
+if (empty($api)) {
     list_verify($type);
 }