fixed some bugs

fixed some bugs.
issue:
table structure:
CREATE TABLE `table1` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(25),
  `rating` int DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM;

if use insert function:
$db=new Sparrow();
...
$db->from('table1')->insert('name'=>'my name','rating'=>null)->execute(); // fail!!!
echo $db->from('table1')->insert('name'=>'my name')->sql();
// output: insert table1 (name,rating) values ('my name',''); // rating init empty string!
after fixed output sql statement:
insert table1 (name,rating) values ('my name',null)