fix(#808): only add error to response when it is set

[zoey-kaiser]

🔗 Linked issue

Potentially closes #808

❓ 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

After being redirected to the login page, the URL looks like this: http://localhost:3000/login?callbackUrl=http://localhost:3000/dashboard&error=undefined

The error query parameter is undefined, and I would like to change this to a specific error message or remove it altogether.

We wrap the error heading in String() and add a fallback to undefined if it does not exist. However, transferring an error to a string will always result in it being set, as the wrapper undefined in string results in a string that is undefined, which lines up with the path generated!

📝 Checklist

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