Portable stories: Only provide a play function wrapper if it exists

[yannbf]

Closes #

What I did

A recent PR improved the play function in portable stories, however introduced a slight runtime check issue, where if users were to check whether a composed story's play function exists, it would always return true, even if there wasn't one. This PR fixes that, and it also fixes a TypeScript issue where the type of the play function was always defined, even though it could be undefined. This will affect projects that used the play function of a composed story before, and the change is documented in migration notes.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

[JReinhold]

nit: personally I find this syntax more readable in these cases, but it's totally up to you.

Suggested change

	play: story.playFunction
	? ((async (extraContext: ComposedStoryPlayContext<TRenderer, TArgs>) =>
	story.playFunction!({
	...context,
	...extraContext,
	})) as unknown as ComposedStoryPlayFn<TRenderer, Partial<TArgs>>)
	: undefined,
	...(story.PlayFunction && { play: ((async (extraContext: ComposedStoryPlayContext<TRenderer, TArgs>) =>
	story.playFunction!({
	...context,
	...extraContext,
	})) as unknown as ComposedStoryPlayFn<TRenderer, Partial<TArgs>>) })