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/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