Push stability timestamp to indexer only if the timestamp is fully formed

Change-Id: I9d3131d6b7c0f3285aff47a65abf2f1d1639d328
Reviewed-on: http://review.couchbase.org/44029
Reviewed-by: John Liang <johliang00@gmail.com>
Tested-by: John Liang <johliang00@gmail.com>