(PostgreSQL pgcrypto) How to generate blowfish hash without salt?

In PostgreSQL, using the pgcrypto module, I can generate a **slow** hash with blowfish (or any supported algorithm) like this:

crypt('SomeTextHere', gen_salt('bf'));

However, it auto-generates a salt (with gen_salt) and requires it. I don't want to use any salt. I am also not interested in using a normal hashing function (like sha256). How can I generate a **slow** hash without a salt?