ui: date-picker - the invalid status is shown even if the picker is pristine

[christophercr]

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 a date picker is initialized as empty and with the required validator, the picker is shown as invalid after being initialized.

Expected behavior

The date picker should be shown as invalid only when the user has interacted with the picker or he has changed its value.

Minimal reproduction of the problem with instructions

• Add a stark-date-picker element to your template.
• Provide a FormControl to the date picker. Initialize the form control with undefined and add the required validator to it.
• Run the application. The date picker will be immediately shown as invalid and with the "required" error message.

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

Better UX.

Environment

Angular version: X.Y.Z
Stark version: 10.0.0-beta.6, 10.0.0-beta.7