Fix problem on task creation with chrome for french browser.

With Chrome on windows 8 french $_SESSION["dol_tz_string"] is "Paris/_Madrid", that is not a valid parameters for $localtz = new DateTimeZone($default_timezone);

In all case, if  $localtz is empty we should initialze with something, else new DateTime(null,$localtz); will trhow an execption with second paramters cannot be NULL.