Fix Notification Error on empty watch list

[mikebgrep]

With this PR, I fix the issue when there are no watched pages on the home screen and the user try to update and test the notification option from settings. Previously error was displayed, now the test notification work.
Notification test was passed also I test manual with empty watch and available watch.

  File "/app/changedetection/changedetectionio/flask_app.py", line 232, in decorated_view
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/app/changedetection/changedetectionio/flask_app.py", line 549, in ajax_callback_send_notification_test
    watch_uuid = random.choice(list(datastore.data['watching'].keys()))
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/random.py", line 347, in choice
    raise IndexError('Cannot choose from an empty sequence')

[dgtlmoon]

Thanks!

This cant be merged without a test, so i've added a test in another branch and merged it there

[mikebgrep]

Hi, great. I was thinking of looking if the test is needed, but when I rethink, I decide that maybe it is covered.

[dgtlmoon]

this broke a lot of stuff that was not tested for, for example, sending a test notification when in restock mode

[mikebgrep]

Is not sending when restock is available? I can take a look tomorrow if you are not in a rush.