Don't fallback to dvla_organisation if letter branding unset

The template preview app now accepts a null value for the `filename` 
parameter. If a service doesn't have a letter branding option set, 
previously we defaulted to their dvla_organisation (probably HM 
Government). Now, we pass through None, so that we generate letters 
without any logo or branding.

tests/app/celery/test_letters_pdf_tasks.py

@@ -182,8 +182,7 @@ def test_create_letters_pdf_sets_technical_failure_max_retries(mocker, sample_le
     mock_update_noti.assert_called_once_with(sample_letter_notification.id, 'technical-failure')
 
 
-# We only need this while we are migrating to the new letter_branding model
-def test_create_letters_gets_the_right_logo_when_service_has_dvla_logo(
+def test_create_letters_gets_the_right_logo_when_service_has_no_logo(
         notify_api, mocker, sample_letter_notification
 ):
     mock_get_letters_pdf = mocker.patch('app.celery.letters_pdf_tasks.get_letters_pdf', return_value=(b'\x00\x01', 1))
@@ -194,7 +193,7 @@ def test_create_letters_gets_the_right_logo_when_service_has_dvla_logo(
     mock_get_letters_pdf.assert_called_once_with(
         sample_letter_notification.template,
         contact_block=sample_letter_notification.reply_to_text,
-        filename=sample_letter_notification.service.dvla_organisation.filename,
+        filename=None,
         values=sample_letter_notification.personalisation
     )
 

tests/app/service/test_rest.py

@@ -141,7 +141,7 @@ def test_get_service_by_id(admin_request, sample_service):
     assert 'branding' not in json_resp['data']
     assert json_resp['data']['dvla_organisation'] == '001'
     assert json_resp['data']['prefix_sms'] is True
-    assert json_resp['data']['letter_logo_filename'] == 'hm-government'
+    assert json_resp['data']['letter_logo_filename'] is None
 
 
 @pytest.mark.parametrize('detailed', [True, False])
@@ -294,10 +294,10 @@ def test_create_service_with_no_domain_doesnt_set_letter_branding(admin_request,
     json_resp = admin_request.post('service.create_service', _data=data, _expected_status=201)
 
     assert json_resp['data']['letter_branding'] is None
-    assert json_resp['data']['letter_logo_filename'] == 'hm-government'
+    assert json_resp['data']['letter_logo_filename'] is None
 
 
-def test_get_service_by_id_returns_letter_branding_not_dvla_organisation(
+def test_get_service_by_id_returns_letter_branding(
         client, sample_service
 ):
     letter_branding = create_letter_branding(

tests/app/template/test_rest.py

@@ -982,6 +982,7 @@ def test_preview_letter_template_by_id_valid_file_type(
                 'postcode': 'A_POST',
             }
             assert post_json['date'] == '2012-12-12T00:00:00'
+            assert post_json['filename'] is None
             assert base64.b64decode(resp['content']) == content