fix: support Pydantic MISSING sentinel in ops.Relation.save

[dimaqq]

Support Pydantic's experimental MISSING sentinel field types and values.

ops.Relation.load() does the right thing already.
ops.Relation.save() needs a small fix to erase missing fields

Check list:

• core code change
• unit tests pass
• ruff format
• docs update
• test on real Juju --> chore: test Pydantic MISSING dimaqq/hexanator#7
• separate new test case
• make the new test conditional on being able to import MISSING
• (whatever reviews bring)

Fixes #2299

[james-garner-canonical]

The core code change looks good to me, as does the to-do list.

[james-garner-canonical]

The implementation looks good based on our team discussion.

The addition of a class using this sentinel to the tests is great, ensuring that we don't crash on MISSING anymore. I think it would be a good idea to add a test that explicitly asserts that MISSING doesn't get serialized as well.

Are you still planning to add some real Juju tests, per the checklist in the PR description? I'm not sure if they're necessary, but I'm definitely not opposed to us adding more integration tests.

[tonyandrewmeyer]

I remain unconvinced that there's a need to support this use-case, and also that "MISSING" should mean "REMOVE", but the PR does implement the team decision.

I do think we should explicitly document the behaviour in a comment - maybe also in the reference doc as well (it feels like that should be expected if we want charms to rely on this behaviour).

[james-garner-canonical]

I'm happy with the new behaviour and with the test coverage.

[benhoyt]

Seems reasonable to me, thanks.