Date Pickers inconsistent behavior when no fallback

[mhenkens]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[x] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

When no date fallback set in dateFormat for the datepicker, and the user insert invalid value we get an inconsistent behavior:
The date input containt text and a RequiredField error appear. This is because the date return to ngModel is undifined

Expected behavior

Get an invalid date error

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

Environment

Angular version: X.Y.Z
Stark version: X.Y.Z


Browser:
- [ ] Chrome (desktop) version XX
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ ] Firefox version XX
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX
 
For Tooling issues:
- Node version: XX  
- Platform:  

Others: