Fix 22030: Object Relations order changed upon concurrent edition

Link: https://jira.ez.no/browse/EZP-22030

## Description
See steps to reproduce in the issue, they are very clear (thanks @jjCavalleri )

I checked the behavior of other eZDataType and noticed that this method is supposed to return `false` if the post variable is not available. Since in this scenario this is not, I added the return statement and it fixed the problem.

## Tests
Manual tests