fix(AuthForm): ensure header is shown with leading slot

[rodriciru]

🔗 Linked issue

Resolves #5404

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Added a new condition to show the leading slot if not title or no desc or no icon or no heading prop

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[rodriciru]

Sorry about the failed spec. I will try to update it, but will be my first time

[rodriciru]

I hope I update correctly the snapshot

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@5405

commit: 8b932fa

[benjamincanac]

Thanks 😊