results.yaml contains null fields

[happz]

Namely start-time, end-time, and duration: all are Optional[str], and can be None - subresults, for example - but documentation does not mention they can exist but be unset.

Personally, I would omit them from the file completely, current serialization does not allow that, but to_minimal_spec() should. It would also remove the ugly __class__ field from results.yaml, and better align results with the fact that they are, indeed, following a specification.

[thrix]

From triage meeting: rather drop duringto_minimal_spec method