Merge branch '6533-domain-module-prevents-subdomains-to-be-added-for-domains-that-exist-as-website' into 'develop'

Resolve "Domain module prevents subdomains to be added for domains that exist as website"

Closes #6533

See merge request ispconfig/ispconfig3!1764

Author: Till Brehm

interface/lib/classes/tools_sites.inc.php

@@ -145,13 +145,9 @@ function convertClientName($name){
 	}
 
 	/* TODO: rewrite SQL */
-	function getDomainModuleDomains($not_used_in_table = null, $selected_domain = '') {
+	function getDomainModuleDomains($not_used_in_table = null, $selected_domain = null) {
 		global $app;
 
-		// Fix a null value passed in by the create new forms, which lack an id.
-		if ($selected_domain == null) {
-			$selected_domain = '';
-		}
 		$sql = "SELECT domain_id, domain FROM domain WHERE";
 		if ($not_used_in_table) {
 			if (strpos($not_used_in_table, 'dns') !== false) {