Edited Issue: Should get null instead of "NULL" for varchar DEFAULT NULL [156]

SQL
---
```SQL

CREATE TABLE [dbo].[CC]
(
[Id] int NOT NULL IDENTITY(1,1) PRIMARY KEY,
[Desc] varchar(MAX) NULL DEFAULT NULL ,
[X1] varchar(255) NULL
)
GO

```
but got
```C#

// CC
public class Cc
{
public int Id { get; set; } // Id (Primary key)
public string Desc { get; set; } // Desc
public string X1 { get; set; } // X1

public Cc()
{
Desc = "NULL";
}
}
```
instead of `Desc = null;`

Tested in MSSQL & POCO 2.16.0/1