Docs: Fix MDX Stories block tag-filtering behavior

[shilman]

Closes #27143

What I did

Made a sneaky change to the tag-based filtering of autodocs introduced in 8.1, which was an unintentional breaking change for users who used the Stories doc block in MDX pages.

Now if NONE of the stories have the autodocs tag (which is only possible in an MDX page), it will show ALL of the stories in the Stories block.

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

You can check the added stories:

• No Autodocs
• Some Autodocs

You can also reproduce this in a sandbox by adding Button.mdx and playing around with the tags in Button.stories.ts:

import { Meta, Stories } from '@storybook/blocks'
import * as ButtonStories from './Button.stories'

<Meta of={ButtonStories} />

# Button stories

<Stories />

🦋 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>

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