fix: allow session launcher parameters to be reset

[olevski]

Allows the API to accept None as input for args, command and the session launcher resource class ID so that they can be reset to their defaults in patch endpoints.

[olevski]

This change is part of the following stack:

• [REBASE only]: Migrate notebooks and add new amalthea #418
  • fix: allow session launcher parameters to be reset #434 ◀
    • fix: docker image check endpoint and gitlab authn #439
      • fix: add hibernation and deletion time in status #440

_{Change managed by git-spice.}

[Panaetius]

We had to solve the same issue in renku-python in the past, but we solved it with a specific object instance, not a type: https://github.com/SwissDataScienceCenter/renku-python/blob/develop/renku/domain_model/constant.py#L23

The benefit is that you can just use object id equality instead of isinstance checks, so it functions much more like regular None does than just a type. E.g. https://github.com/SwissDataScienceCenter/renku-python/blob/develop/renku/domain_model/constant.py#L23

I.e. there's only ever a single NO_VALUE object that contains nothing, except for the python object id, which is the same.

This is a common idiom for sentinel values, there is a PEP to have better support for sentinels but it's still a draft: https://peps.python.org/pep-0661/

The downside is that the object() approach doesn't have a nice repr, though not sure if we care about that.

You can use a class with a repr, but then it's important that you use the type as the value, not an instance, to have it behave like None does. This is discussed in a bit more detail in https://python-patterns.guide/python/sentinel-object/ and https://www.revsys.com/tidbits/sentinel-values-python/

In any case, we should use one of the more standard approaches.

[olevski]

@Panaetius ok I will switch to something like the example from renku python.