title: Selectfield in sencha touch doesn't respect an empty value
body: <p>I've noticed a problem with ST2 and selectfield pickers.  I'm testing this on Desktop browser and tablet and both seem to show the same problem.</p>
  
  <p>The problem seems to stem from having form data that is empty or uninitialised.
  My example is a user logs into their account and needs to set their marital status. As this has never been set before the backing store model is actually 'null' for their marital status. When they click the picker, the pick for some reason picks the first item in the checklist automatically. This is evident by the check-mark on the right side of the item.  The 2nd side-effect of this is, if you then select the first item, ST2 doesn't see this as an item change and so doesn't then propagate the selection change back to the form.</p>
  
  <p>Is this is a bug? How do I get round this problem?</p>
  
  <pre><code>Ext.define('Gender', {
      extend: 'Ext.data.Model',
      config: {
          fields: [
              {name: 'Id', type: 'int'},
              {name: 'ItemName',  type: 'string'}           
          ]
      }
  });
  
  Ext.define('Details', {
      extend: 'Ext.data.Model',
      config: {
          fields: [       
              {name: 'Gender',  type: 'int'}           
          ]
      }
  });
  
  var myGenderStore = Ext.create('Ext.data.Store', {
      model: 'Gender',
      data : [
          {Id: 1, ItemName: 'Male'},
          {Id: 2, ItemName: 'Female'}
      ]   
  });
  
  var myDetailsStore = Ext.create('Ext.data.Store', {
      model: 'Details',
      data : [
          { Gender: null }        
      ]   
  });
  
  var p = Ext.create('Ext.form.Panel', {
      fullscreen: true,
      items: [
          {
              xtype: 'fieldset',
              title: 'Select',
              items: [
                  {
                      xtype: 'selectfield',
                      label: 'Choose one',
                      displayField: 'ItemName',
                      valueField: 'Id',
                      store: myGenderStore,
                      name: 'Gender'
                  }
              ]
          }
      ]
  });
  
  p.setRecord(myDetailsStore.getAt(0))
  Ext.Viewport.setActiveItem(p);
  // notice the picker has 'Male' selected even though the backing store for the Gender field is null
  // also, we want to select Male from the list, but this isn't reflected on the form
  // run below command in console window after selecting 'Male' even though it is selected and it shows null
  // It only seems to like changes to the value as selecting female works. If then select Male from Female this also works.
  p.getValues().Gender;
  </code></pre>