[QGIS-Server] WFS Filter : send empty data instead of XML error when no feature found

Bug #8123
When a getFeature request has a FILTER parameter, and when no data
corresponds, QGIS answer sometimes with :
<ServiceExceptionReport version="1.3.0">i
  <ServiceException code="RequestNotWellFormed">No feature found error messages: .</ServiceException>
</ServiceExceptionReport>
It would be better to send back an empty GML (Or GeoJSON) instead.