replace `gpac.empty` with `None`

[dave-doty]

Josh did this for ppsim. I think it's as simple as rewriting the methods __rshift__ and __rrshift__:

• https://github.com/UC-Davis-molecular-computing/ppsim-rust/blob/8f164ff2544529445dc6f11e86a02862d2ed98e8/python/ppsim/crn.py#L231
• https://github.com/UC-Davis-molecular-computing/ppsim-rust/blob/8f164ff2544529445dc6f11e86a02862d2ed98e8/python/ppsim/crn.py#L237
• https://github.com/UC-Davis-molecular-computing/ppsim-rust/blob/8f164ff2544529445dc6f11e86a02862d2ed98e8/python/ppsim/crn.py#L307