Avoid false values in calendar views

'false' is displayed when no value is assigne to a field declared in a calend.

Impacted versions:

8.0, master
Steps to reproduce:

Create a fresh database, create a new customer and set his fiscal position.
Create a quotation, select the customer
Current behavior:

File "/Users/keje/src/odoo/addons/account/partner.py", line 107, in get_fiscal_position
return part.property_account_position.id
NameError: global name 'part' is not defined

Expected behavior:

No error
Merge pull request #1083 from jkei/patch-3

Avoid false values in calendar views