fix(dns): Avoid PHP errors in DNS plugin detected by tests

When creating a zone a non-set var was used as an array

issue #5716

dns/admin/dns/class_dnsZone.inc

@@ -336,6 +336,8 @@ class DnsRecordsAttribute extends DialogOrderedArrayAttribute
         }
       }
       $this->initialReverseZones = $reverseZones;
+    } else {
+      $this->initialReverseZones = array();
     }
     sort($this->value);
   }