feat: add empty slot fallback content info (#8302)

[jdtjenkins]

Description (required)

This PR adds an example of how passing empty slots causes that slot's default content to not render.

Related issues & labels (optional)

• Closes #8302

For a new/changed feature in an upcoming Astro release?

No

First-time contributor to Astro Docs?

Yes :3

Discord: Jumper#8174