title: remove empty option from ng-select
body: <p>How can I avoid the empty value in the ng-select.
  My intention is that whenever I have a boolean property in the component set to true - there is only one record in the validFiltersData - then I want the drop down to be selected on it and disabled, so the user cannot select the empty option.</p>
  
  <pre><code>&lt;ng-container *ngFor="let validFilter of validFiltersData; let i = index"&gt;
    &lt;div *ngIf="validFilter.Values &amp;&amp; validFilter.Values.length &gt; 0" 
         class="filter col-md-4 reportFilter"&gt;
      &lt;ng-select [items]="validFilter.Values"  bindLabel="Text" 
                  [(ngModel)]="validFilter.selectedValue" 
                   [ngClass]="{'input-md': true,  'ng-invalid': 
                                                    !validFilter.IsValid }"&gt;
      &lt;/ng-select&gt;
    &lt;/div&gt;
  &lt;/ng-container&gt;
  </code></pre>