Products.CMFActivity: Fix username display in failure notifications.

This was broken by:
  commit f363ac65
  Author: Vincent Pelletier <vincent@nexedi.com>
  Date:   Wed Mar 23 15:55:46 2022 +0900

      Products.CMFActivity.ActivityTool: Store user object in activity.
as user_name becomes always None.
24 jobs for fix/ERP5Site_reindexLatestIndexedObjects in 0 seconds
Status Job ID Name Coverage
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