title: ngBootstrap for Angular 4 DatePicker in a Reactive Form is Always Required
body: <p>Using a reactive form I've added the ngbDatePicker:</p>
  
  <pre><code>&lt;input type="text"
          id="business_incorp_date"
          class="form-control"
          formControlName="business_incorp_date"
          ngbDatepicker
          #incorporatedDatePicker="ngbDatepicker"
          (click)="incorporatedDatePicker.toggle()"
          readonly&gt;
  </code></pre>
  
  <p>For a form model of:</p>
  
  <pre><code>this.form = this.formBuilder.group({
    id: [
      0, [Validators.required]
    ],
  
    // ... removed for brevity
  
    business_type: [
      '', [Validators.required]
    ],
    business_incorp_date: [
      '', [FormValidators.date] // &lt;-- NOT REQUIRED BY MODEL
    ],
    business_desc: [
      '', [Validators.required]
    ]
  
    // ...
  });
  </code></pre>
  
  <p>But, if the field is empty it is still ngInvalid for the <code>ngbDateFormat</code>:</p>
  
  <pre><code>{ "ngbDate": { "invalid": { "year": null, "month": null, "day": null } } }
  </code></pre>
  
  <p>Anyone know how to make the DatePicker field allow for an empty string?</p>