code: public String readString(String attribute)
      {
          final String value = getValue(attribute);
          if (NULL.equals(value))
          {
              return null;
          }
          return value;
      }
docs: Read a string. If the read string is equal to {@link #NULL}, <code>null</code> will be returned instead.
  
  @param attribute The string name (must not be <code>null</code>).
  @return The string value.
  @throws LionEngineException If error when reading.