HSETNX can lead coding type change even when skipped due to NX (#4615)

1. Add one key-value pair to myhash, which the length of key and value both less than hash-max-ziplist-value, for example:
>hset myhash key value
2. Then execute the following command
>hsetnx myhash key value1 (the length greater than hash-max-ziplist-value)
3. This will add nothing, but the code type of "myhash" changed from ziplist to dict even there are only one key-value pair in "myhash", and both of them less than hash-max-ziplist-value.