code: public static Object toVoid(Object o) throws ExpressionException {
  	if (o == null) return null;
  	else if (o instanceof String && o.toString().length() == 0) return null;
  	else if (o instanceof Number && ((Number) o).intValue() == 0) return null;
  	else if (o instanceof Boolean && ((Boolean) o).booleanValue() == false) return null;
  	else if (o instanceof ObjectWrap) return toVoid(((ObjectWrap) o).getEmbededObject(null));
  	throw new CasterException(o, "void");
      }
docs: cast a value to void (Empty String)
  
  @param o
  @return void value
  @throws ExpressionException