acknowledge block empty in event details if there are no acknowledgements

acknowledge block is empty (missing the table) if there are no acknowledges for an event. broken by :

r22573 | eduards | 2011-10-20 15:41:34 +0300 (Thu, 20 Oct 2011) | 1 line

• ZBX-4153 fixed links to acknowledge and returning back to requester page

(i wanted to report this one a long time ago, but only now got to finding out the actual breakage revision )