enh: show empty task files in GUI/TUI

[Selene ToyKeeper]

Currently (tkdo 0.7), the GUI and TUI don't display empty task files at all. They should probably make sure at least one line is displayed, by adding a placeholder when the file would otherwise be invisible.

This causes problems because without at least one item, the GUI and TUI are unable to do anything to the file, such as adding tasks. This can happen either when a project has no active tasks, or when a user first runs TKDO and doesn't have any tasks yet.

So, if a task file is empty or contains only completed/snoozed items (and those aren't currently visible), add a single item called '(none)' or even just the title of the task file with nothing after it.