Fix empty view layout

Steps to reproduce:

• Create a custom document type with a custom schema.
• Navigate to Designer and generate all missing layouts.
• View layout is empty:

  <!--
  `nuxeo-contract-view-layout`
  @group Nuxeo UI
  @element nuxeo-contract-view-layout
  -->
  <dom-module id="nuxeo-contract-view-layout">
    <template>
      <style>
        *[role=widget] {
          padding: 5px;
        }
      </style>
    </template>
  
    <script>
    Polymer({
      is: 'nuxeo-contract-view-layout',
      behaviors: [Nuxeo.LayoutBehavior],
      properties: {
  
        /**
           * @doctype Contract
           */
        document: {
          type: Object,
        },
  
      }
    });
    </script>
  </dom-module>