Raise on empty array given to `del`

Following from #74, DEL should have the same treatment:

MockRedis:

    > $redis.del []
    => 0

redis-rb:

    > $redis.del []
    => Redis::CommandError

Change-Id: I9289f6de7f465376aa8fc3c67b7a4e93fb6b9e50
Reviewed-on: https://gerrit.causes.com/33232
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>