trade: fix a wrong proxy listbox usage in "Trade Model Lines" listbox

Listbox supports setting '' (empty string) as an URL column to disable the link, but None was never really supported, even though it worked in the old UI. When setting using the "key | value" form of formulator, if value is empty, it's not None but an empty string. This is also what we test in portal_tests/listbox_zuite/testUrlColumns