fix(nuxt): make slot response fallback property undefineable

[huang-julien]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ 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

Hey 👋 this PR makes the fallback field for slots in the NuxtIslandResponse undefined by default.

Before that, there was a nullish operator which was triggering esbuild warning when building (in dev).

I think it's better to move the nullish operator to an OR (||) and make the field undefineable instead

PS: is there a way to detect that in the CI ?

📝 Checklist

• I have linked an issue or discussion.
• I have added tests (if possible).
• I have updated the documentation accordingly.

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