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

Fix issue when DNS zone import fails silently with OK message if $TTL is missing from the input file because TTL can not be null when saving SOA to database.
Solution: if TTL is null then simply set it to 1 day - 86400 seconds (as per RFC 1912 recommendations).
I encountered few services that export a dns zone without global ttl set (ex: GoDaddy).

interface/web/dns/dns_import.php

@@ -611,7 +611,7 @@ if(isset($_FILES['file']['name']) && is_uploaded_file($_FILES['file']['tmp_name'
 	$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);