Update parameter validation to treat the case where "of" is None

[wreise]

Reference issues/PRs

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description
The bug #589 is caused by the fact that the validation is attempted, even thought ref_of is None.

Screenshots (if appropriate)

Any other comments?

Checklist

• I have read the guidelines for contributing.
• My code follows the code style of this project. I used flake8 to check my Python changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed. I used pytest to check this on Python tests.

[wreise]

Locally, tests fail on gtda/homology/tests/test_cp_transform.

[ulupo]

Thanks so much!