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