Merge branch '6599-issue-in-ajax_get_json-php-when-dkim_strength-0' into 'develop'

Resolve "Issue in ajax_get_json.php when $dkim_strength = 0"

Closes #6599

See merge request ispconfig/ispconfig3!1823

Author: Till Brehm

interface/web/mail/ajax_get_json.php

@@ -51,7 +51,7 @@
 	unset($rec);
 	$mail_config = $app->getconf->get_server_config($server_id, 'mail');
 	$dkim_strength = $app->functions->intval($mail_config['dkim_strength']);
-	if ($dkim_strength=='') $dkim_strength = 2048;
+	if ($dkim_strength == '' || $dkim_strength == 0 ) $dkim_strength = 2048;
 
 	$rnd_val = $dkim_strength * 10;
 	$app->system->exec_safe('openssl rand -out ../../temp/random-data.bin '.$rnd_val.' 2> /dev/null');