Non default search and replace of omitted optional value

[andersonbrands]

No description provided.

[florisla]

To recap, version v0.0 (0.0-prod) is not properly bumped to v0.1-dev. That's a serious bug.

While investigating your fix, I've found an alternative way to fix this: drop the found_required variable in version_part.py VersionConfig._serialize().

But I'm not sure which of the two approaches is most robust.

I'll add a test case where also the serialization {major} is valid.
Then, test all combinations of config file version (0, 0.0 or 0.0-prod) with three different files, each
having a longer serialize -- these should bump to 1, 1.0 and 1.0-dev respectively.