Fix EZP-23322: validate if required object relation list is empty

JIRA: https://jira.ez.no/browse/EZP-23322

This fixes a regression introduced in EZP-23028, causing a required objectrelationlist attribute to be accepted.
CS:

```php
if (
    $http->postVariable( $postVariableName ) != array( "no_relation" )
    && count( $http->postVariable( $postVariableName ) ) > 0
)