Data access enhancements

This PR enhances the data access functions, to provide 'category', 'folder' and 'brand' substitutions when a product (or content) is defined (as Thelia 1 does ^_^ ).
Minor enhancements (CS, comments, style) are made on DataAccessFunctions.php class.

On the asset's side, a message is logged if the generated url is empty, of if the 'file' parameter of assets Smarty function (stylesheets, javascripts, images) is missing.
NotFoundHttpException is thrown by dataAccessWithI18n() in this case.

I've added the return clause (which was missing) to return a empty string instead of null when the product_id request parameter is not defined.