feat: Show Niri/window component when workspace is empty

[diego-velez]

Match behavior to Hyprland/window where when workspace is empty users can still rewrite the title.

For example, currently when a Niri workspace is empty the component is hidden, but on Hyprland/window you can do something like this:

{
    "modules-center": [
        "hyprland/window",
    ],
    "hyprland/window": {
        "rewrite": {
            "": "No window in workspace"
        }
    }
}

In Hyprland/Waybar, this would show No window in workspace when the workspace is empty.