Pass undefined workflow params as null.

frontend/src/js/page/wb-launch-workflow-page.js

@@ -215,7 +215,7 @@ class WBLaunchWorkflowPage extends Component {
         const { isFile } = inputSpecInfo(main.inputs.find(a => (a.id === k)));
         val = uuidsToCwl(val, isFile);
         k = k.split('/').slice(1).join('/');
-        inputs[k] = val;
+        inputs[k] = (val === undefined ? null : val);
       } catch (exc) {
         errors.push('Error parsing ' + k + ': ' + exc.message);
       }