Check if a project can be password protected with an empty password.

Currently the code checks if the password is not empty. Maybe change self._use_pw_hash from a bool to some sort of enum.

Search the Python code for the check that rejects an empty password and inspect every use of self._use_pw_hash. First determine how password protection is represented for empty and non-empty passwords, then verify the resulting vbaProject.bin behavior for both cases. Done means the supported behavior is explicit and covered by an appropriate test.

Written by the indexing model from the issue text.