add test to ensure that updating other things doesnt affect sms sender

app/models.py

@@ -188,7 +188,7 @@ class Service(db.Model, Versioned):
     created_by_id = db.Column(UUID(as_uuid=True), db.ForeignKey('users.id'), index=True, nullable=False)
     reply_to_email_address = db.Column(db.Text, index=False, unique=False, nullable=True)
     letter_contact_block = db.Column(db.Text, index=False, unique=False, nullable=True)
-    sms_sender = db.Column(db.String(11), nullable=False, default=lambda: current_app.config['FROM_NUMBER'])
+    sms_sender = db.Column(db.String(11), nullable=True, default=lambda: current_app.config['FROM_NUMBER'])
     organisation_id = db.Column(UUID(as_uuid=True), db.ForeignKey('organisation.id'), index=True, nullable=True)
     organisation = db.relationship('Organisation')
     dvla_organisation_id = db.Column(

tests/app/service/test_rest.py

@@ -1735,3 +1735,19 @@ def test_update_service_does_not_call_send_notification_when_restricted_not_chan
 
     assert resp.status_code == 200
     assert not send_notification_mock.called
+
+
+def test_update_service_works_when_sms_sender_is_null(sample_service, client, mocker):
+    sample_service.sms_sender = None
+    data = {'name': 'new name'}
+
+    resp = client.post(
+        'service/{}'.format(sample_service.id),
+        data=json.dumps(data),
+        headers=[create_authorization_header()],
+        content_type='application/json'
+    )
+
+    assert resp.status_code == 200
+    # make sure it wasn't changed to not-null under the hood
+    assert sample_service.sms_sender is None