set sms_sender to be 'GOVUK' if not otherwise specified

this is a precursor to making the column non-nullable

tests/app/service/test_rest.py

@@ -144,6 +144,7 @@ def test_get_service_by_id(client, sample_service):
     assert json_resp['data']['organisation'] is None
     assert json_resp['data']['branding'] == 'govuk'
     assert json_resp['data']['dvla_organisation'] == '001'
+    assert json_resp['data']['sms_sender'] == 'GOVUK'
 
 
 def test_get_service_by_id_should_404_if_no_service(notify_api, notify_db):
@@ -213,6 +214,7 @@ def test_create_service(client, sample_user):
     assert json_resp['data']['email_from'] == 'created.service'
     assert not json_resp['data']['research_mode']
     assert json_resp['data']['dvla_organisation'] == '001'
+    assert json_resp['data']['sms_sender'] == 'GOVUK'
 
     auth_header_fetch = create_authorization_header()
 

tests/app/v2/notifications/test_post_notifications.py

@@ -34,7 +34,7 @@ def test_post_sms_notification_returns_201(notify_api, sample_template_with_plac
             assert resp_json['reference'] == reference
             assert resp_json['content']['body'] == sample_template_with_placeholders.content.replace("(( Name))", "Jo")
             # conftest fixture service does not have a sms sender, use config default
-            assert resp_json['content']['from_number'] == notify_api.config["FROM_NUMBER"]
+            assert resp_json['content']['from_number'] == 'GOVUK'
             assert 'v2/notifications/{}'.format(notification_id) in resp_json['uri']
             assert resp_json['template']['id'] == str(sample_template_with_placeholders.id)
             assert resp_json['template']['version'] == sample_template_with_placeholders.version