Add test cases for commands run against empty zsets

This disproves the need for #47.

Change-Id: I17662e8b76c9bb925a0cb4af1cc53d06b6a619e6
Reviewed-on: https://gerrit.causes.com/33204
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>