fix(back): avoid php notice when poller has not timezone (#6031)

www/class/centreonGMT.class.php

@@ -356,8 +356,9 @@ class CentreonGMT
     public function getHostCurrentDatetime($host_id, $date_format = 'c')
     {
         $locations = $this->getHostLocations();
+        $timezone = isset($locations[$host_id]) ? $locations[$host_id] : '';
         $sDate = new DateTime();
-        $sDate->setTimezone(new DateTimeZone($this->getActiveTimezone($locations[$host_id])));
+        $sDate->setTimezone(new DateTimeZone($this->getActiveTimezone($timezone)));
         return $sDate;
     }