Removing last element in a zset should remove the key itself (e.g. affects zrem + exists)

[TysonAndre]

Similar to #927 but for zsets instead of hashes

In ioredis-mock, exists returns 1, but a real redis server would return 0

zremrangebyscore (etc) are also affected.

127.0.0.1:6379> zadd myset 1 key
(integer) 1
127.0.0.1:6379> exists myset
(integer) 1
127.0.0.1:6379> zrem myset key
(integer) 1
127.0.0.1:6379> exists myset
(integer) 0