Merge branch 'patch-1' into 'stable-3.1'

Fix issue when DNS zone import fails silently with OK message if $TTL is missing…

See merge request ispconfig/ispconfig3!695

Author: Till Brehm

interface/web/dns/dns_import.php

@@ -611,7 +611,7 @@
 	$retry = $soa['retry'];
 	$expire = $soa['expire'];
 	$minimum = $soa['minimum'];
-	$ttl = $soa['ttl'];
+	$ttl = isset($soa['ttl']) ? $soa['ttl'] : '86400';
 	$xfer = '';
 	$serial = $app->functions->intval($soa['serial']+1);
 	//print_r($soa);