zendframework/zf2 — pull request #6871 by marc-mabe (2014-11-14 08:16:59+00:00)
Title: Memcached returns FALSE on failure

... so it's not required to check for NULL, too.
The note Memcached returns NULL was a miss information in changelog of memcached-2.0

PS: It's not a bug - it's an improvement as the result will be same ;)