feat(Input/Textarea): nullify model modifier

[Gerbuuun]

🔗 Linked issue

resolves #2281

❓ 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

Adds model modifier which makes the input return null instead of a falsy value (most importantly undefined and "")
See linked issue for explanation.
This is a follow-up of the #2275 PR

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.
  Modifiers are not documented right now, should I add something about them?

[romhml]

I've wrapped my mind around it, and I think the implementation is simple enough to integrate. Thanks!

[benjamincanac]

@romhml Shouldn't this be done on all form components for consistency? 🤔