code: protected void removeEmptyDirectories(File outputDirectory)
      {
          if (outputDirectory.exists())
          {
              for (File file : outputDirectory.listFiles(new EmptyDirectoryFilter()))
              {
                  file.delete();
              }
          }
      }
docs: Deletes any empty directories under the output directory.  These
  directories are created by TestNG for its own reports regardless
  of whether those reports are generated.  If you are using the
  default TestNG reports as well as ReportNG, these directories will
  not be empty and will be retained.  Otherwise they will be removed.
  @param outputDirectory The directory to search for empty directories.