CBQE-1547: add tests for null values

Change-Id: I24220fd3d4dccc361bfac36011cc4c7e9af700fe
Reviewed-on: http://review.couchbase.org/29631
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>