UI: Update Button types to allow for no children on iconOnly buttons

[cdedreuille]

What I did

I improved the types of the new Button component to allow for iconOnly buttons to not have children.

How to test

1. Run this branch locally
2. Test the Button anywhere with <Button iconOnly icon={<><>} />
3. You should not see any typescript errors

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

• 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.

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

🦋 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>

[kasperpeulen]

I think this should not be optional in this case

[cdedreuille]

Thanks. I made it mandatory.

[kasperpeulen]

LGTM!