Return empty list when there are no users for the service.

Added a test for when there are no users for the service.
Added a test_url_for - do we want to add this test and use url_for in our tests?
Or explictly write the url in the test?

tests/app/service/test_url_for.py

@@ -0,0 +1,40 @@
+import uuid
+
+from flask import url_for
+
+service_id = str(uuid.uuid4())
+
+
+def test_url_for_get_services(notify_api):
+    with notify_api.test_request_context():
+        url = url_for('service.get_services')
+        assert str(url) == '/service'
+        url_with_user_id = url_for('service.get_services', user_id=1)
+        assert str(url_with_user_id) == '/service?user_id=1'
+
+
+def test_url_for_get_service_by_id(notify_api):
+    with notify_api.test_request_context():
+        url = url_for('service.get_service_by_id', service_id=service_id)
+        assert str(url) == '/service/{}'.format(service_id)
+
+        url_with_user_id = url_for('service.get_service_by_id', service_id=service_id, user_id=1)
+        assert str(url_with_user_id) == '/service/{0}?user_id={1}'.format(service_id, 1)
+
+
+def test_url_for_create_service(notify_api):
+    with notify_api.test_request_context():
+        url = url_for('service.create_service')
+        assert str(url) == '/service'.format(service_id)
+
+
+def test_url_for_update_service(notify_api):
+    with notify_api.test_request_context():
+        url = url_for('service.update_service', service_id=service_id)
+        assert str(url) == '/service/{}'.format(service_id)
+
+
+def test_url_for_renew_api_key(notify_api):
+    with notify_api.test_request_context():
+        url = url_for('service.renew_api_key', service_id=service_id)
+        assert str(url) == '/service/{}/api-key'.format(service_id)