Set var: Optional[Type] to None via CLI

[anqixu]

Let me start by saying that SimpleParsing is an amazingly useful library!

This question has been asked previously, but I haven't seen a direct line of discussion.

Consider the following use case:

@dataclass
class ProcessFlow:
  do_step: Optional[FlowParams] = FlowParams()

$ ./run.py --do_step=None  # or =null

The above syntax has the downside that, given var: Optional[str], it would cause a contention between passing in the string keyword is used to represent None, versus the string literal "None". In this edge case, we can consider preferring e.g. None over "None".

What do you guys think? @lebrice?

Many thanks!