Send api the broadcast provider restriction of 'all', not 'None'

We're replacing the value of None with the value of all. API has been
updated to accept both values
(https://github.com/alphagov/notifications-api/pull/3233/commits/1767535defd117e1d39d3ab911f01477787eef65)
so this change starts sending notifications-api the value of "all".

app/main/forms.py

@@ -2334,7 +2334,7 @@ class ServiceBroadcastAccountTypeField(GovukRadiosField):
             split_values = self.data.split("-")
             self.service_mode = split_values[0]
             self.broadcast_channel = split_values[1]
-            self.provider_restriction = split_values[2] if len(split_values) == 3 else None
+            self.provider_restriction = split_values[2] if len(split_values) == 3 else 'all'
 
 
 class ServiceBroadcastAccountTypeForm(StripWhitespaceForm):

tests/app/main/views/test_service_settings.py

@@ -5524,9 +5524,9 @@ def test_get_service_set_broadcast_account_type_has_radio_selected_for_broadcast
 @pytest.mark.parametrize(
     'value,service_mode,broadcast_channel,allowed_broadcast_provider',
     [
-        ("training-test", "training", "test", None),
+        ("training-test", "training", "test", "all"),
         ("live-test-vodafone", "live", "test", "vodafone"),
-        ("live-severe", "live", "severe", None),
+        ("live-severe", "live", "severe", "all"),
     ]
 )
 def test_post_service_set_broadcast_account_type_posts_data_to_api_and_redirects(