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

@@ -27,8 +27,9 @@
             %li
               = link_to dashboard_todos_path, title: 'Todos', data: {toggle: 'tooltip', placement: 'bottom', container: 'body'} do
                 = icon('bell fw')
-                %span.badge.todos-pending-count
-                  = todos_pending_count
+                - unless todos_pending_count == 0
+                  %span.badge.todos-pending-count
+                    = todos_pending_count
             - if current_user.can_create_project?
               %li
                 = link_to new_project_path, title: 'New project', data: {toggle: 'tooltip', placement: 'bottom', container: 'body'} do