Can't search for participants without selecting an event.

When I try to search for participants of all events (/civicrm/event/search page), it triggers an error :
'' is not a valid option for field id

This is since CRM-15803, as it changes the input value of the event reference field to an array() with '' as value, which should be an empty array if there is no value.
I have this issue for the 4.6.0 and 4.6.2 release

Strangely this doesn't trigger an error on the d46 and the dmaster demo sites (see http://d46.demo.civicrm.org/civicrm/event/search), so maybe this is fixed somewhere else.

The attached PR fixes this issue (if it isn't fixed somewhere else)