Bug: Incorrect error checking of bcrypt hashes

The method `BcryptHasher::make()` does a strict check for `false` to see if the function call `password_hash()` went wrong. However, this function may also return `null` on error. In that case, the return value will be accepted despite the error.

The strict comparison generally makes no sense, because no falsy return value could possibly be valid. Anything other than a string with 60 ASCII characters means that the hashing has failed.