CBQE-0: fix rebalance progress tests if getting empty stats

Change-Id: Id82349ed82fa72a97c41a153695aba8803f6e913
Reviewed-on: http://review.couchbase.org/26590
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>