fix(nuxt): check for uid before replacing data-island-uid attribute in island

[huang-julien]

🔗 Linked issue

danielroe/roe.dev@b33100c

❓ 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

Hi 👋 this PR fix an issue where nested islands can be buggy and render invalid html (double attributes).

This is due to the fact that we were replacing all data-island-uid without checking if those were already set.

I'll add a test later.

📝 Checklist

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

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