Show workspaces on all outputs if persistent_workspaces value is empty

[PucklaJ]

This would make the behavior consistent with sway/workspaces and wlr/workspaces. A config like this:

"hyprland/workspaces": {
    "disable-scroll": true,
    "active-only": false,
    "all-outputs": false,
    "on-click": "activate",
    "sort-by-number": true,
    "persistent_workspaces": {
      "1": [],
      "2": [],
      "3": [],
      "4": [],
      "5": []
    }
  }

Would show workspaces 1 - 5 on all outputs.