types(controls): allow null for modelValue

[andre-brdoch]

Description

With this PR, null is being added as a valid modelValue type to all controls.

Before it needed to be either the type that is reasonable for the control in question (eg. string for HstText), or undefined. However, authors of components (for which they are writing stories) might choose to allow null as a value for their props.

Controls should not have much opinion about how authors choose to type their components, but instead make it easy and seamless to write stories.

Let me know if you have any questions or disagree 😊

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

type adjustment

Before submitting the PR, please make sure you do the following

• If it's a new feature, provide a convincing reason to add it. Ideally, you should open a suggestion issue first and have it approved before working on it.
• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

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[andre-brdoch]

It complains about the PR title, types is not supported. I assume the same rules apply as for the commit message rules? According to those, types is valid:
https://github.com/histoire-dev/histoire/blob/main/.github/commit-convention.md

/^(revert: )?(feat|fix|docs|dx|refactor|perf|test|workflow|build|ci|chore|types|wip|release|deps)((.+))?: .{1,50}/