Do not treat emtpy string ('') as null

In Postgresql, there is a strong distinction between an empty string and a null.

When loading datas from the DB, the convertFromPg transform '' to a null value.

let's imagaine this : 
```SQL
Create Table MyTBL( id serial, a text not null, b text not null);
Insert into MyTBL(1, 'Hello', '');
```
Then in Pomm
```PHP
$map = $conn->getMapFor('\Hello\World\MyTbL');
$obj = $map->findByPk(array('id' => 1)));
$obj->setA('World');
$map->saveOne($obj);
```
This will give you and error about B violating the not null constraint


Let's change this...

what do you think?