Task endpoint should work if a task has a missing DocumentRoute

In some cases DocumentRoute documents can be deleted while Task documents referring to them still exist. If this case happens, the task endpoint will not return data when invoked for the user, whether the user has valid tasks or not.

The task endpoint should fail gracefully when this happens – the tasks which are valid should as results.

To reproduce:

• Start a process on a document
• Get the ID of the task assigned to the user
• Verify that the new task shows up in the results of the task endpoint curl -u Administrator:Administrator localhost:8080/nuxeo/api/v1/task
• Look in the Task document for the DocumentRoute id curl -u Administrator:Administrator localhost:8080/nuxeo/api/v1/id/19f5bff4-f562-485c-94d8-1d3c85700dc5 -H 'X-NXproperties: *' – look for routeInstanceDocId in nt:taskVariables
• Delete the DocumentRoute document curl -u Administrator:Administrator localhost:8080/nuxeo/api/v1/id/19f5bff4-f562-485c-94d8-1d3c85700dc5 -X DELETE
• Task endpoint request will fail curl -u Administrator:Administrator localhost:8080/nuxeo/api/v1/task