title: ExtJS store sync send empty string instead of null
body: <p>I have a <code>store</code> with <code>model</code>, and this <code>model</code> have <code>fields</code> like below:</p>
  
  <pre><code>{
      name: "name",
      type: "string",
      allowNull: false,
      defaultValue: null
  }
  </code></pre>
  
  <p><strong>Problem is:</strong> When I update <code>textfield</code> with name on empty string, I send "" instead of <code>null</code></p>
  
  <p><code>textfield</code> have <code>allowNull: true</code></p>
  
  <p>How to send <code>null</code>?</p>