Addon-controls: Support undefined option for optional values

[lifeiscontent]

Describe the bug

should have an option to set an arg to undefined if its optional

To Reproduce
Steps to reproduce the behavior:

1. Create a UI Component + Story
   2.Add an optional prop
2. See that there is no option to set the arg to undefined in storybook controls / docs

Expected behavior
I'd expect that if an arg is optional, it should have undefined as a value in args.

Screenshots

Code snippets
If applicable, add code samples to help explain your problem.

System
Please paste the results of npx sb@next info here.

Additional context
Related bug that was fixed previously: #4487

[stale]

Hi everyone! Seems like there hasn't been much going on in this issue lately. If there are still questions, comments, or bugs, please feel free to continue the discussion. Unfortunately, we don't have time to get to every issue. We are always open to contributions so please send us a pull request if you would like to help. Inactive issues will be closed after 30 days. Thanks!

[tmeasday]

@lifeiscontent see #14579.

The TLDR is:

1. In code: You can set an arg to undefined, purely by not setting the arg in the story (we no longer set the arg to its default value).
2. In controls: You can reset all arg values to the story's initial values (so undefined if the story didn't set it) with the reset button
3. You cannot set a control to undefined if the story set a value.

This was a decision discussed at length with @shilman and @domyen, it seemed a good compromise between over-complicating things and letting a user do what they want to do.