Handle test cases with nullptr as cookie for get_log_info
Handle test cases with nullptr as cookie for get_log_info

The DCP tests in ep-engine contains tests which use nullptr
for the cookie.

Change-Id: Ic3988b47c602fd646b9bfdd8b15c6789ea3562ec
Reviewed-on: http://review.couchbase.org/75848
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Daniel Owen <b3fc0e1a51f252986e60be7da121b74d3ac6845f@couchbase.com>

daverigby/memcached,couchbase/memcached,daverigby/memcached,daverigby/memcached,couchbase/memcached,couchbase/memcached,daverigby/kv_engine,daverigby/memcached,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,couchbase/memcached