Because of ERP5Type.UnrestrictedMethod, 'System Processes' can own objects.
Such objects can be proxy-role'd scripts, and proxy-role mechanism
triggers many users look-ups (each time security is evaluated, which is
virtually every getattr). Each such lookup will do a query for 'System
Processes' user, which will (hopefully) find nothing anyway.
So special-case 'System Processes' when looking by user_id by skipping
the search altogether (enforcing the inability to locate this user,
consistently with Zope assumptions, and consistently with previous
behaviour).