[-] FO : fix contact-form id_order input when customer is not logged

Some popular module always set the isLogged variable in smarty (pm_advancedtopmenu, pm_adsandslideshow, favourite...), and it can be set to false, or 0. When the customer is not logged, the contact form doesn't render properly, since it there is no "input" that shows. 
That's why instead of "!isset", putting "empty" gives the expected output. (it checks for both isset and false !).