title: QStyleSheet property for QPushButton with empty text
body: <p>I'm attempting to do something like this: </p>
  
  <pre><code>QPushButton[text=""] {
      background-color: rgb(255, 0, 0);
  }
  
  QPushButton {
      background-color: rgb(0, 0, 0);
  }
  </code></pre>
  
  <p>Basically, some of my QPushButtons have text, and some don't, and I want to style the ones that do differently from the ones that do not. </p>
  
  <p>This works if I switch on an actual string value, ie</p>
  
  <pre><code>QPushButton[text="Ok"] {
      background-color: rgb(255, 0, 0);
  }
  </code></pre>
  
  <p>Then any buttons that say "Ok" have a red background. But what do I pass in for a button with no text?</p>