EZP-21768: eZCollaborationItemGroupLink: Changed wrong var comparison to assignments

When `$userID` is not provided (or `false` is given) to the methods `fetch()` and `fetchList()` the current user's ID should be used. Currently, `$userID` would remain `false` because `$userID` is compared to `eZUser::currentUserID();` instead of assigning it to the former.

https://jira.ez.no/browse/EZP-21768

Cheers
:octocat: Jérôme