inactive-windows-transparency: Fix crashes when there is no focused window

[CromFr]

I don't know exactly when this happens (maybe on lock screen?), but sometimes sway does not report any focused window, and this results in a crash in inactive-windows-transparency.py.

'NoneType' object has no attribute 'workspace'
Traceback (most recent call last):
  File "/usr/share/sway-contrib/inactive-windows-transparency.py", line 88, in <module>
    ipc.main()
    ~~~~~~~~^^
  File "/usr/lib/python3.13/site-packages/i3ipc/connection.py", line 514, in main
    raise loop_exception
  File "/usr/lib/python3.13/site-packages/i3ipc/connection.py", line 497, in main
    while not self._event_socket_poll():
              ~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib/python3.13/site-packages/i3ipc/connection.py", line 477, in _event_socket_poll
    raise e
  File "/usr/lib/python3.13/site-packages/i3ipc/connection.py", line 474, in _event_socket_poll
    self._pubsub.emit(event_name, event)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/i3ipc/_private/pubsub.py", line 28, in emit
    s['handler'](self.conn, data)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/usr/share/sway-contrib/inactive-windows-transparency.py", line 24, in on_window
    focused_workspace = focused.workspace()
                        ^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'workspace'

[CromFr]

Damn I just noticed somebody already fixed it in a PR: #56

The implemented solution is a bit different though. From what I understand, @chamlis solution skips setting windows transparency if nothing is focused, while my solution basically sets all windows to transparent

Feel free to close this PR if don't like my implementation :)

[OctopusET]

Every PRs are welcomed, thanks.
Though, original behavior wasn't like this.
619783a
But I like your solution too.
Can you fix the conflict?
I think I can try add this at some point.