[FIX] account: return format of function field

When no result is found on the function field 'invoice' (account.move.line), instead of returning {move_id: (False, '')}, return {move_id: False} (expected for m2o fields)
Fixes #2138, opw 613096
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32