Allow empty changes to build-team-manifests, for forced builds

Change-Id: I941d31cf5f36d5835676b025c3fe5e61f4339f08
Reviewed-on: http://review.couchbase.org/44614
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>