Yup. It is a common practice, an empty flat, only walls.
It's the normal practice. Some years ago they tried selling apartments with bathrooms and kitchens (and maybe flooring too) already installed (pod klucz) and the reaction.... was not positive. Polish buyers seem to like making decisions about kitchen and bathroom and flooring for themselves.
I just checked and you can find apartments ready to move into (again look for the term 'pod klucz') but you'll have to do some looking.
article in Polish about the phenomenon...