feat(forms): allow null as initial value

[Gerbuuun]

🔗 Linked issue

Resolves #1210

❓ 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 model modifier to UInput and UTextarea to make the value return null if the field value is falsy (because initial values may not be a string?).
I'm not actually sure if the type change is allowed like this.
Does the native element support an initial value of null? I suppose it is similar to undefined...

📝 Checklist

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

[Gerbuuun]

I've created a new feature request for the model modifier (#2281). I'll update this PR to only include the type changes. After that we can discuss the modifier.

[romhml]

Thanks :)

[benjamincanac]

@romhml Do we need to do this on v3 as well?

[romhml]

Yes, I'll take care of it.

[Gerbuuun]

Now actually trying this (@nuxt/[email protected]) it does not seem to work...
Is there a difference with component types between the playground and actual package?

Can someone confirm this works?