Don't show the Todo count if there aren't any Todos.

When there are 0 Todos, the Todos icon still had the blue badge with a "0" in it, which made me think I had a new Todo left. This makes it less annoying.

Follow-up on !4077.

app/views/layouts/header/_default.html.haml

@@ -27,6 +27,7 @@
             %li
               = link_to dashboard_todos_path, title: 'Todos', data: {toggle: 'tooltip', placement: 'bottom', container: 'body'} do
                 = icon('bell fw')
+                - unless todos_pending_count == 0
                   %span.badge.todos-pending-count
                     = todos_pending_count
             - if current_user.can_create_project?