CBQE-0: set priority N/A to reporting json for empty priority

Change-Id: I587233fbc990baa2605418c1fb1716ab331c9100
Reviewed-on: http://review.couchbase.org/37711
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>