@Override
  public Predicate toPredicate(String prop, String val) throws InvalidQueryException {
    if (val != null) {
      throw new InvalidQueryException("'isEmpty' operator shouldn't have a right operand but one exists: " + val);
    }
    return new CategoryIsEmptyPredicate(prop);
  }
Operator that is used to determine if a category is empty, meaning that it doesn't
contain any properties.
ambari-server/src/main/java/org/apache/ambari/server/api/predicate/operators/IsEmptyOperator.java