querycmd/scanner: Fix nil key for low/high scan keys

Change-Id: Icd0bbfe05aaef5174822102d4a20532236c7b357
Reviewed-on: http://review.couchbase.org/44009
Reviewed-by: Sriram Melkote <siri@couchbase.com>
Tested-by: Sriram Melkote <siri@couchbase.com>