[FIX] digest: check field existance

This commit fixes the following bug:
* On a new DB, install account
* Settings > Technical > Email > Digest emails
* Open Weekly Digest, set Next Send Date to current day
* Settings > Technical > Scheduled Actions > Digest Emails
* Run manually

AttributeError: 'res.company' object has no attribute
'resource_calendar_id'

The field resource_calendar_id is added in the module resource.
The module digest does not have resource in its dependency.
In stable version, have a silent fallback on UTC if the field is not present.
The right dependency should be added in master.

closes odoo/odoo#28244

addons/digest/models/digest.py

@@ -158,7 +158,8 @@ class Digest(models.Model):
 
     def _compute_timeframes(self, company):
         now = datetime.utcnow()
-        tz_name = company.resource_calendar_id.tz
+        # TODO remove hasattr in >=saas-12.1
+        tz_name = hasattr(company, "resource_calendar_id") and company.resource_calendar_id.tz
         if tz_name:
             now = pytz.timezone(tz_name).localize(now)
         start_date = now.date()