title: :not(:empty):before pseudo-classes / elements combination
body: <p>I would like to display a label only if the content is there.</p>
  
  <p>This does not work:
  <a href="https://jsfiddle.net/9g0amsh1/" rel="nofollow noreferrer">https://jsfiddle.net/9g0amsh1/</a></p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.views-field-field-documents .field-content:not(:empty):before {
    content: "Documents associés : ";
    float: left;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="views-field views-field-field-documents"&gt;&lt;div class="field-content"&gt;
  &lt;/div&gt;&lt;/div&gt;
      
  &lt;div class="views-field views-field-field-documents"&gt;&lt;div class="field-content"&gt;
      &lt;span class="file file--mime-application-pdf file--application-pdf"&gt;&lt;a href="/system/files/article/documents/boarding-pass.pdf" type="application/pdf; length=117264"&gt;boarding-pass.pdf&lt;/a&gt;&lt;/span&gt;
  &lt;/div&gt;&lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>