@JZL003 solution is great, but parentFolder is not actually used, so code could be simplified further, such as:

function recurseFolder(myFolder) {
  var childfolders = myFolder.getFolders();   // get the list of sub folders in the root of that folder
  while (childfolders.hasNext()) {            // count the sub folders
    recurseFolder(childfolders.next());
  }
  var hasFile = myFolder.getFiles().hasNext();      // get the list of files in the root of that folder
  var hasFolder = myFolder.getFolders().hasNext();  // get the list of files in the root of that folder
  if (!hasFile && !hasFolder) {            
    myFolder.setTrashed(true);
  }             
}