fix: do not go in transfer playback condition when asking for category

[maxenced]

When asking for a category with some call like :

action: spotcast.start
data:
  category: 0JQ5DAqbMKFI3pNLtYMD9S
  spotify_device_id: xxxxxxxxxxxxxxxxxxxxxxx

the part of code in start_casting which check for player transfer did not check if category was empty, hence result of above call was only a transfer (if needed), but category is never changed.