NaN from zero length sessions

https://github.com/lamethods/labook-code/blob/a2aa3b6b3bd1e83b69b853620205c3c22e1313c6/ch07-prediction/feature_creation.R#L84C1-L84C87

I'm getting some NaN values in the session_len_entropy field. 
I think it's because I have some data with 0 length session. 

I'm not sure if it's a bug, but I'll try removing sessions with 0 length. 