[FIX] web: FieldSelection on m2o: unset value

When the value of a many2one field with widget selection was unset,
it's value wasn't actually set to false in the BasicModel. It was
set to a virtual id like 'virtual_294' instead. This caused a
traceback when this value was sent to the server, obviously.

This was for example the case of the Register Payment form view
(accessible from a customer invoice), when the user selected a
payment journal, and then unselected it (the virtual id was then
sent to the onchange RPC).