title: Jenkinsfile archive step: allow missing artifacts
body: <p>Under the regular UI defined jobs I can select <em>"Do not fail build if archiving returns nothing"</em>. How do I achieve the equivalent in pipeline code? </p>
  
  <p>E.g. currently I have something along these lines:</p>
  
  <pre><code>archive 'screenshots/**', 'build/test/results/*.xml'
  </code></pre>
  
  <p>and it is OK if there are no screenshots since there would be none if all tests pass.</p>