Redis RedisModule_ZsetRem can lead to "empty" zset keys

A module using only the low level zset API can easily arrive at empty keys - an unheard of thing in Redis. Of course, the author could always check the size of the zset after zrem, and delete the key if it is 0... yuck.

@antirez @dvirsky Unless I'm doing something wrong, I think this needs to be handled by Redis.

P.S. This quote is so funny :)

Empty keys will be handled correctly by doing nothing.

Minimal reproducible code example (starting to implement ZPOP):

#include "redismodule.h"

int test(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
    RedisModuleKey *key = RedisModule_OpenKey(ctx, argv[1], REDISMODULE_READ | REDISMODULE_WRITE);

    int type = RedisModule_KeyType(key);
    if (REDISMODULE_KEYTYPE_EMPTY == type) {
        RedisModule_CloseKey(key);
        RedisModule_ReplyWithNull(ctx);
        return REDISMODULE_OK;
    }

    if (REDISMODULE_KEYTYPE_ZSET != type)
    {
        RedisModule_CloseKey(key);
        RedisModule_ReplyWithError(ctx, REDISMODULE_ERRORMSG_WRONGTYPE);
        return REDISMODULE_OK;
    }

    RedisModule_ZsetFirstInScoreRange(key, REDISMODULE_NEGATIVE_INFINITE, REDISMODULE_POSITIVE_INFINITE, 0, 0);

    RedisModuleString *ele = RedisModule_ZsetRangeCurrentElement(key, NULL);
    RedisModule_ZsetRangeStop(key);

    int deleted;
    RedisModule_ZsetRem(key, ele, &deleted);

    RedisModule_CloseKey(key);
    RedisModule_ReplyWithString(ctx, ele);
    return REDISMODULE_OK;
}

int RedisModule_OnLoad(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
    REDISMODULE_NOT_USED(argv);
    REDISMODULE_NOT_USED(argc);

    if (RedisModule_Init(ctx,"module",1,REDISMODULE_APIVER_1)
        == REDISMODULE_ERR) return REDISMODULE_ERR;

    if (RedisModule_CreateCommand(ctx,"test",
        test,"write",1,1,1) == REDISMODULE_ERR)
        return REDISMODULE_ERR;

    return REDISMODULE_OK;
}

Terminal session:

127.0.0.1:6379> TYPE z
none
127.0.0.1:6379> TEST z
(nil)
127.0.0.1:6379> ZADD z 0 a 1 b
(integer) 2
127.0.0.1:6379> TYPE z
zset
127.0.0.1:6379> TEST z
"a"
127.0.0.1:6379> TYPE z
zset
127.0.0.1:6379> TEST z
"b"
127.0.0.1:6379> TYPE z
zset
127.0.0.1:6379> ZCARD z
(integer) 0
127.0.0.1:6379> EXISTS z
(integer) 1
127.0.0.1:6379> TEST z
Could not connect to Redis at 127.0.0.1:6379: Connection refused