code: private String getCertFromHeader(final HttpServletRequest request) {
          val certHeaderValue = request.getHeader(sslClientCertHeader);
          if (StringUtils.isBlank(certHeaderValue)) {
              return null;
          }
  
          if ("(null)".equalsIgnoreCase(certHeaderValue)) {
              return null;
          }
          return StringUtils.trim(certHeaderValue);
      }
docs: Get certificate from header or return null.
  HTTPD mod_header writes "(null)" when the ssl variable is not filled
  so that is treated as if the header were not present or blank.
  
  @param request HTTP request object
  @return Base64 encoded certificate