commit.post() with no tags or category will return permlink too short RPC error

If no tags=[] or category is especified the RPC node returns: 

```
http_client.py", line 278, in call
    raise RPCError(detail)
steembase.exceptions.RPCError: assert_exception from xxx.xxx.xxx.xxx
(Assert Exception:permlink.size()  > STEEM_MIN_PERMLINK_LENGTH && permlink.size() < STEEM_MAX_PERMLINK_LENGTH: 
Permlink is not a valid size.) in broadcast_transaction
``` 

