Bugfix: SinglePartition may not include coordinator.

It is possible that Index topology for a feed request may not include
coordinator endpoint. Projector should gracefully handle this situation.

Change-Id: I23cb95fc552d806f4ad22d70418c3efca2ed1774
Reviewed-on: http://review.couchbase.org/43438
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>