Core: Fix falsey default args handling

[tmeasday]

Issue: #12767

What I did

Check if default value is undefined, rather than falsey before excluding it.

How to test

• Is this testable with Jest or Chromatic screenshots?

Yes, see test

• Does this need a new example in the kitchen sink apps?

I think probably not. I don't think these things are picked up by chromatic anyway so don't end up being very useful to have edge cases, unless they are hard to reproduce.

[shilman]

💯

[shilman]

@tmeasday can you take a look at the chromatic changes? probably ok but i'd like a second opinion

[tmeasday]

@shilman -- I approved the first two changes -- this example was a component with a children prop with default value null. I think given current semantics it is correct that that prop should get passed to the component with null value [1]

I'm not sure about the other 8 changes -- are they present on next?

[1] Arguably we shouldn't be setting args to the default value from the component? That might be a discussion for another time.

[shilman]

@tmeasday I accepted a few more changes (zoom changes are test flake, parameters change is from another PR that slipped in). The remaining ones are all due to this PR I think.

[tmeasday]

@shilman OK those stories highlighted a second bug that was obscured by this bug, I think the examples I added in the other PR (#13919) would have made that more obvious.

[tmeasday]

@shilman I am going to merge this into #13919

There's too many overlapping concerns, it's silly to resolve them separately