Fixed EZP-22466 advanced object relation list editing

> Replaces #913 
> Fixes http://jira.ez.no/browse/EZP-22466

Fixes the condition at the top of `eZObjectRelationList::validateObjetAttributeHTTPInput()`.
It should now only skip validation if the attribute isn't required AND doesn't contain any relation.

The issue was a regression from https://jira.ez.no/browse/EZP-18790. The bug fixed in this issue
was tested, and no regression was found.