Fix Undefined property: $mappers

[wordish]

Import Express and you'll get this error from the XHR request to /dashboard/system/migration/import/validate_batch:

Undefined property: PortlandLabs\\Concrete5\\MigrationTool\\Batch\\Validator\\Pipeline\\Stage\\ValidateExpressAttributesStage::$mappers

Sample XML to reproduce the error:

<concrete5-cif version="1.0">
  <expressentries>
    <entry entity="supporter">
      <attributes>
        <attributekey handle="supporter_name">
          <value>Friday Design</value>
        </attributekey>
        <attributekey handle="supporter_subtitle">
          <value/>
        </attributekey>
        <attributekey handle="supporter_link">
          <value>https://www.example.com</value>
        </attributekey>
        <attributekey handle="supporter_level">
          <value>
            <option>300</option>
          </value>
        </attributekey>
      </attributes>
    </entry>
  </expressentries>
</concrete5-cif>