Update Python API to support optional creator and layer

[beojan]

Creators and layers (once #817 is merged) are now optional in C++ and JSON but still manadatory in Python.

Originally raised by @chatgpt-codex-connector[bot] in #817 (comment)

[wlav]

The required fixes (and a few other things to make testing easier) are here: https://github.com/wlav/phlex/tree/python-optional-layer

Note that that branch will not work with the changes from 817 branch.