Check `output_path` exists before reading in backup

[curz46]

• If the output_path does not yet exist, pressing Apply will throw an AttributeError because load_text_file(...) returns null.
• With this change, we check for the existence of the file before trying to load the backup. If it does not exist, we fallback to an empty array.

Traceback (most recent call last):
  File "/usr/lib/python3.11/site-packages/nwg_displays/main.py", line 519, in on_apply_button
    apply_settings(display_buttons, outputs_activity, outputs_path, use_desc=config["use-desc"])
  File "/usr/lib/python3.11/site-packages/nwg_displays/main.py", line 834, in apply_settings
    backup = load_text_file(outputs_path).splitlines()
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'splitlines'

Steps to reproduce

1. Delete $HOME/sway/outputs or equivalent output_path for the window manager in use.
2. Launch nwg-displays via command-line and press apply.
3. Observe that the UI does not update. In the console, observe the AttributeError due to file not found.