Fallback user should also be used when user can not be assigned to issues

Prerequisite
Backbone is configured with a User passthrough mapping for the assignee field and a fallback user is configured.

Current situation
Backbone takes the fallback user only if the received user does not exist. If it can not be assigned to issues, the sync will fail.

Improvement
The fallback user should also be used if the received user can not be assigned to this issue.