properly support watchdog 4+ where events always have dest_path

[evgeni]

Pull Request Checklist

• I’ve read the guidelines for contributing.
• I updated AUTHORS.txt and CHANGES.txt (if the change is non-trivial) and documentation (if applicable).
• I tested my changes.

Description

Since watchdog 4 all events have the dest_path property,
but that property can be empty, thus breaking nikola auto:

ERROR: asyncio: Task exception was never retrieved
future: <Task finished name='Task-57' coro=<CommandAuto.reload_page() done, defined at nikola/plugins/command/auto/__init__.py:427> exception=ValueError('no path specified')>
Traceback (most recent call last):
  File "nikola/plugins/command/auto/__init__.py", line 440, in reload_page
    p = os.path.relpath(event_path, os.path.abspath(self.site.config['OUTPUT_FOLDER'])).replace(os.sep, '/')
  File "/usr/lib64/python3.8/posixpath.py", line 453, in relpath
    raise ValueError("no path specified")
ValueError: no path specified

[evgeni]

See gorakhargosh/watchdog@41fca1e for the change in watchdog

[evgeni]

Fixes #3766, I guess

[Kwpolska]

Thanks for contributing!