Avoid false values in calendar views

'false' is displayed when no value is assigne to a field declared in a calendar views.

Impacted versions:

8.0, master
Steps to reproduce:

Create a custom calendar view with a custom model
Declare a non mandatory field in the calendar view
Create a new object without any value for this non mandatory field
Display the calendar view

Current behavior:

'false' is displayed in the calendar item.

Expected behavior:

Nothing displayed if the field has no value.