title: PrimeNg-Button : button not showing fa icon if label is not provided
body: <p>I want to show button with icon and without label. 
  and i am trying with this</p>
  
  <pre><code>&lt;button type="text" pButton icon="fa-angle-left"&gt;&lt;/button&gt;
  </code></pre>
  
  <p>but it is not showing fa icon, and displaying blank button.
  if i try giving label, then it will show icon as well as label.</p>
  
  <p>don't know if i am doing anything wrong.</p>