fix(nuxt): allow serialising undefined refs

[danielroe]

🔗 Linked issue

resolves #20787 (comment)

❓ 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 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

This handles the case of undefined empty refs, which otherwise were not correctly being handled as JSON.stringify was of course not quite up to the job.

📝 Checklist

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