title: placeholder not shown in ngselect
body: <p>I'm working with ng-select in my angular form ( <a href="https://github.com/ng-select/ng-select" rel="noreferrer">https://github.com/ng-select/ng-select</a>) the code that I'm using :</p>
  <pre><code>&lt;ng-select
              formControlName=&quot;model&quot;
              name=&quot;model&quot;
              id=&quot;add_sheet_model&quot;
              [items]=&quot;modelItems&quot;
              [multiple]=&quot;false&quot;
              [searchable]=&quot;false&quot;
              bindLabel=&quot;value&quot;
              bindValue=&quot;id&quot;
              placeholder=&quot;select model&quot;
            &gt;
            &lt;/ng-select&gt;
  </code></pre>
  <p>the problem is that the placeholder is not displayed when I put [multiple] = &quot;false&quot; but when I put [multiple] = &quot;true&quot; the placeholder is displayed and I don't want my ngselect to be multiple.</p>
  <p>Ps: I'm working with angular 10 and ngselect 5.0.3.</p>
  <p>Any idea ?</p>