code: private String getOptionalParameter(String keyword,
                                          Map<String, String> parms) {
          if (!parms.containsKey(keyword)) {
              return null;
          }
  
          String value = parms.get(keyword);
          if (value == null) {
              throw new ValidatorProcessUsageException("If parameter '" + keyword
                      + "' is provided, it must have a value.");
          } else {
              return value;
          }
      }
docs: Get the requested parameter from the map. If it's not there, return null,
  but if it's there with no value, complain.