adopt the changed DB structure [not NULL] to the code

gui/public/client/mail_add.php

@@ -104,7 +104,7 @@ function addMailAccount()
     $mailType = $subId = '';
     $mailTypeNormal = in_array($_POST['account_type'], ['1', '3']);
     $mailTypeForward = in_array($_POST['account_type'], ['2', '3']);
-    $mailQuotaLimitBytes = NULL;
+    $mailQuotaLimitBytes = 0;
 
     // Check for username
     $username = mb_strtolower(clean_input($_POST['username']));

gui/public/client/mail_edit.php

@@ -80,7 +80,7 @@ function client_editMailAccount()
     $mainDmnProps = get_domain_default_props($_SESSION['user_id']);
     $password = $forwardList = '_no_';
     $mailType = '';
-    $mailQuotaLimitBytes = NULL;
+    $mailQuotaLimitBytes = 0;
 
     if (!preg_match('/^(.*?)_(?:mail|forward)/', $mailData['mail_type'], $match)) {
         throw new Exception('Could not determine mail type');