Docs: Don't allow passing of={} with undefined prop

[tmeasday]

Closes #21441

What I did

As it is easy in MDX to write (e.g.) <Story of={Something.Undefined} /> and get no prompt from the editor, we need to disambiguate from <Story /> where we fallback to the primary.

Here we throw if we detect this situation.

How to test

• See stories.
• Try yourself in a sandbox with some of the blocks.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[yannbf]

@tmeasday seems like the change is causing breakages in many stories like so:

[tmeasday]

Good thing we have tests huh @yannbf!

[JReinhold]

Looks good! It's a shame this can't be generalized into useOf, but that would require passing in props to useOf which wouldn't be worth it I think.