Don't try to retrieve tasks without route instance

As a Nuxeo user, I can't retrieve tasks through the REST API in several cases :

• if there's at least one task without process id.
  The error results in a http error 500 which is caused by a NullPointerException in DocumentRoutingServiceImpl#getTasks

• if there's at least one task referencing a missing route instance
  The error results in a http error 404 which is caused by a DocumentNotFoundException in DocumentRoutingServiceImpl#getTasks

REST API call :

http://localhost:8080/nuxeo/site/api/v1/task