title: Tree add child with empyty root give my root = 0, NestedTreeRepository search null
body: <p>i have entity</p>
  
  <pre><code>/**
   * @Gedmo\Tree(type="nested")
   * @ORM\Table(name="mKeyword")
   * use repository for handy tree functions
   * @ORM\Entity(repositoryClass="KeywordRepository")
   */
  class Keyword {
  
  /**
       * @Gedmo\TreeRoot
       * @ORM\Column(name="root", type="integer", nullable=true)
       */
      private $root;
  </code></pre>
  
  <p>when i add keyword </p>
  
  <pre><code>$node = new Keyword();
  
          $node-&gt;setTitle($data['nm']);
  
              $parent = $this-&gt;get_node($parentId);
  
              $node-&gt;setParent($parent);
  
          $em = $this-&gt;getDoctrine()-&gt;getManager();
  
          $em-&gt;persist($node);
  </code></pre>
  
  <p>i have in database root_id = 0 </p>
  
  <p>when i use </p>
  
  <pre><code>$this-&gt;db = $this-&gt;getDoctrine()-&gt;getRepository("MeaKeywordsBundle:Keyword")
  $tree = $this-&gt;db-&gt;getChildren($node,true);
  </code></pre>
  
  <p>db is 
  Gedmo\Tree\Entity\Repository\NestedTreeRepository</p>
  
  <p>i get empty results. i see in query i have where root_id is null. But in database i have 0 </p>