It's a bit sad to have this in `document_show_tab_idle()` which is not a name particularly specific to the startup -- although it is in practice.
More importantly, this doesn't handle the case where you close everything, create a new document (untitled), *do not touch it*, and open a real one: the untitled gets closed, and the real one doesn't get notified. I'm not sure what the right fix is here, because it's not as simple as changing the condition in `show_tab_cb()`: doing it in `document_show_tab()` works, but it also notifies in other situations that are not required (like e.g. *save all*). I didn't yet find a good solution (but didn't spend so much time on this either)