API: Fix validation issue in sendmail rules

src/pretix/plugins/sendmail/api.py

@@ -49,7 +49,7 @@ class RuleSerializer(I18nAwareModelSerializer):
             if not full_data.get('send_date'):
                 raise ValidationError('send_date is required for date_is_absolute=True')
         else:
-            if not all([full_data.get(k) for k in ['send_offset_days', 'send_offset_time']]):
+            if not all([full_data.get(k) is not None for k in ['send_offset_days', 'send_offset_time']]):
                 raise ValidationError('send_offset_days and send_offset_time are required for date_is_absolute=False')
 
         if full_data.get('all_products') is False:

src/tests/api/test_sendmail.py

@@ -119,6 +119,23 @@ def test_sendmail_rule_create_min_fail(token_client, organizer, event):
     )
 
 
+@scopes_disabled()
+@pytest.mark.django_db
+def test_sendmail_rule_offset_zero(token_client, organizer, event):
+    create_rule(
+        token_client, organizer, event,
+        data={
+            'subject': {'en': 'meow'},
+            'template': {'en': 'creative text here'},
+            'send_date': '2018-03-17T13:31Z',
+            'send_offset_days': '0',
+            'send_offset_time': '08:40',
+            'date_is_absolute': False,
+        },
+        expected_failure=False,
+    )
+
+
 @scopes_disabled()
 @pytest.mark.django_db
 def test_sendmail_rule_create_minimal(token_client, organizer, event):