Add NULL Values using Self referencing many-to-many Relationship - doctrine-orm

Question

I have two tables:
category
id | name
1 | Programming
2 | Designing
3 | PHP
4 | C#
5 | Adobe Photoshop
category_tree
category_id | parent_id
1 | NULL
2 | NULL
3 | 1
4 | 1
5 | 2
Here is the Entity class:
/**
* #ORM\Entity
* #ORM\Table(name="category")
*/
class Category
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer", nullable=false)
* #ORM\Id
* #ORM\GeneratedValue(strategy="IDENTITY")
*/
protected $id;
/**
* #ORM\Column(type="text", name="name")
*/
protected $name;
/**
* #ORM\Column(type="text", name="description")
*/
/**
* #ORM\ManyToMany(targetEntity="Category")
* #ORM\JoinTable(name="category_tree",
* joinColumns={#ORM\JoinColumn(name="category_id", referencedColumnName="id")},
* inverseJoinColumns={#ORM\JoinColumn(name="parent_id", referencedColumnName="id")}
* )
*/
protected $categories;
public function __construct()
{
$this->categories = new \Doctrine\Common\Collections\ArrayCollection();
}
public function addCategory(Category $category)
{
$this->categories[] = $category;
return $this;
}
public function getCategories()
{
return $this->categories;
}
public function getId()
{
return $this->id;
}
public function setName($name)
{
$this->name = $name;
return $this;
}
public function getName()
{
return $this->name;
}
}
I am using select form element to insert these values in database, When i selected any root category thats work fine , but when i select root or leave blank this can't insert NULL value in "category_tree" table. How can i add NULL value in "category_tree" table?