Fix randomKey pathological case

[moticless]

Randomkey implemented such that if number of keys with expiration is equal to number of keys, then it only retries up-to 100 times to get a random, non-expired, key. If it fails, then it returns expired key. However, if the number of keys with expiration differed from the total number of keys, the function continue retrying until it success. This approach posed potential delays, particularly when a significant portion of keys were expired.

With this change, it will retry unconditionally 100 times.

[sundb]

let's update this comment.

[sundb]

and i found that dbRandomKey may return NULL, but RM_RandomKey doesn't check it.
maybe we can fix it together this PR.

RedisModuleString *RM_RandomKey(RedisModuleCtx *ctx) {
    robj *key = dbRandomKey(ctx->client->db);
    autoMemoryAdd(ctx,REDISMODULE_AM_STRING,key);
    return key;
}