Fix initial validation state

[crobibero]

This fixes an issue where if the user never submits an invalid form the IsValid property is never updated.

Sample repo: https://github.com/crobibero/input-kit-validation

Steps to reproduce:

1. Enter anything into entry
2. Submit
3. Notice "Not valid"
4. Clear entry
5. Submit
6. Notice "Is valid"

[enisn]

Thanks for your contribution, it makes sense and very useful change.