Make SMS prefix setting non-nullable

We have now:
- defaulted new services to start with this column set to `true`
- migrated all preexisting services[1] to have either `true` or `false` set
  for this column

There is no way for a service to switch back from `true`/`false` to
`null`.

This means that we can safely enforce a non-nullable constraint on this
column now.

1. There is a little gotcha: the GOV.UK Notify service still relies on
   the `sms_sender` column. It doesn’t have a row in the
   `service_sms_senders` table. This means the previous migration
   never changed this service’s value for `prefix_sms` from `null`. So
   this commit also changes its value to `False`, so that the rest of
   the migration, of the whole column, is possible.

tests/app/service/test_rest.py

@@ -1551,7 +1551,6 @@ def test_prefixing_messages_based_on_prefix_sms(
 @pytest.mark.parametrize('posted_value, stored_value, returned_value', [
     (True, True, True),
     (False, False, False),
-    (None, None, True),
 ])
 def test_set_sms_prefixing_for_service(
     client,
@@ -1581,6 +1580,18 @@ def test_set_sms_prefixing_for_service(
     assert result['data']['sms_sender'] == current_app.config['FROM_NUMBER']
 
 
+def test_set_sms_prefixing_for_service_cant_be_none(
+    admin_request,
+    sample_service,
+):
+    admin_request.post(
+        'service.update_service',
+        service_id=sample_service.id,
+        _data={'prefix_sms': None},
+        _expected_status=500,
+    )
+
+
 @pytest.mark.parametrize('today_only,stats', [
     ('False', {'requested': 2, 'delivered': 1, 'failed': 0}),
     ('True', {'requested': 1, 'delivered': 0, 'failed': 0})