title: ng-bootstrap ngbDatepicker sets form as invalid if changing from a date to null
body: <p>I'm trying to use bootstrap's ngbDatepicker in an Angular application to support a non-required date field, but if the field comes in with a date, setting the date to null will result in the field/form being marked as invalid.</p>
  
  <p>Using the code below, if <code>note.expires_on</code> initially has a date and is then set to null, the form is marked invalid:</p>
  
  <p><code>&lt;input name="expires_on" [(ngModel)]="note.expires_on" ngbDatepicker #datePicker="ngbDatepicker"&gt;</code></p>
  
  <p>I see from the post below that if the date is initialized as null, then a null value will not result in an invalid form, but in my case, initializing as null doesn't seem like an option.</p>
  
  <p><a href="https://stackoverflow.com/questions/43828287/how-to-mark-ngbdatepicker-valid-in-form-even-when-no-value-is-selected?newreg=7065f041f79444ceb67a041c4f809c17">How to mark ngbDatepicker Valid in form even when no value is selected</a></p>