Fixed not being able to remove all user roles

Fixed not being able to remove all user roles

User roles would only be actioned if they existed in the form request,
hence removal of all roles would have no data to action upon.
This adds a placeholder 0-id role to ensure there is always role data to
send, even when no roles are selected. This field value is latter
filtered out.

Added test to cover.

Likely related to #3922.