How to force the display of an empty field in panels for an editable field?
In your /sites/all/theme/YourTheme/template.php file, you could try using

function YourThemeName_preprocess_content_field(&amp;$variables) {

    if (!isset($variables['field_name']) {
    $variables['field_name'] = ' ';
  }
}


but I don't know if the empty ' ' will show.