Survey: Fix a small bug that leads to a non-empty array

[tbongers-cat]

If $user_ids is not really empty, but contains an empty array, ilObjectLP::resetLPDataForUserIds() will be called with that value. In some situations this leads to ilChangeEvent::hasAccessed() being called with an array instead of a string as $a_user_id, which results in an 'illegal offset type' error.

This only happens if a survey is contained in another object and adds to its learning progress, as ilLPStatusCollection::determineStatus() will be called in this case.

PS: This is already fixed in release_8.