Folder::findOrMake() - Fallback to the assets folder

Fallback to the assets folder if no Folder (DataObject) is found.

See:
CMSFileAddController->getEditForm()
$uploadField->setFolderName('/'); // root of the assets

#2878

- PHP Fatal error: Call to a member function getFilename() on a non-object in UploadField.php on line 1279