Fix EZP-22860: Moving WF event fails if user is not valid

Link: https://jira.ez.no/browse/EZP-22860

## Description
If you remove the user of a validation Workflow (making the WF edit form not valid) and try to move events up or down, you get a fatal error. This fix ensure the events has been retrieved (which is not the case when invalid) before trying to move it.

## Tests
Manual tests