Vue3: Don't map undefined slots

[chakAs3]

Closes #23668,

What I did

I return undefined or null if the slots undefined or null, in the slots mapping function

How to test

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>

[github-actions]

	Fails
🚫	PR is not labeled with one of: ["patch:no","patch:yes"]

Generated by 🚫 dangerJS against 3a7f2c4

[storybook-bot]

Failed to publish canary version of this pull request, triggered by @chakAs3. See the failed workflow run at: https://github.com/storybookjs/storybook/actions/runs/5754725581

[ndelangen]

This PR is likely a duplicate of #23697 @kasperpeulen @chakAs3

I'll close this one, but feel free to re-open if that's incorrect. 🙏