Edit the existing selected mask )or create a new mask) in the File Naming tab and set the mask to:
+ ifnotempty(artist,' - ')
Note, instead of just using artist, I think you want album artist (if it exists) otherwise albums with tracks by different artists will be spilt up. That is the purpose of the
And you probably only want to output a '/' or '-' if you actually have a value. That is the purpose of using ifnotempty function.