Single line patch for support uc_webform_pane

When we use uc_webform_pane to integrate with webform_civicrm, we can't load data from logged user successfully at ubercart checkout page.
After looked into the code, we found this empty check may conflit with ubercart.

This line at function _webform_civicrm_webform_frontend_form_alter of webform_civicrm_forms.inc

  // If this is an edit op, use the original IDs and return
  if (!empty($form['#submission'])) {

should be modify to below to prevent conflit.

  // If this is an edit op, use the original IDs and return
  if (!empty($form['#submission']->sid)) {

This code still at our beta website, need more review and test.