Bug when using optional to dates

Can't remember the exact state that caused this bug, but think it was when using an event with an optional 'to date', in some cases the $date variable would be empty in the dateconnector module around line 152, triggering a php error.

date_timezone_set($date, timezone_open($display_tz));

This patch adds a quick check to the existing one to make sure the $date variable is set.