Fix EZP-23497: allow user de-selection when editing multiplexer workflow

JIRA: https://jira.ez.no/browse/EZP-23497

In the multiplexer workflow, "Users without Workflow IDs" should be de-selectable (no option), unlike the other properties (for which the value `-1` represents "All ...")

Fix: If the corresponding http var is not set, clear this event attribute.

However, `fetchHTTPInput()` was being called on ALL operations, not just on store.
This means that when editing the workflow, the attribute was being reset again (no http post vars)

Solution:
Fetch HTTP input only when storing the workflow.