properly support watchdog 4+ where events always have dest_path

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

CHANGES.txt

@@ -9,6 +9,7 @@ Bugfixes
 
 * Restore `annotation_helper.tmpl` with dummy content - fix themes still mentioning it
   (Issue #3764, #3773)
+* Fix compatibility with watchdog 4
 
 New in v8.3.1
 =============

nikola/plugins/command/auto/__init__.py

@@ -429,7 +429,7 @@ async def reload_page(self, event) -> None:
         # Move events have a dest_path, some editors like gedit use a
         # move on larger save operations for write protection
         if event:
-            event_path = event.dest_path if hasattr(event, 'dest_path') else event.src_path
+            event_path = event.dest_path if (hasattr(event, 'dest_path') and event.dest_path) else event.src_path
         else:
             event_path = self.site.config['OUTPUT_FOLDER']
         p = os.path.relpath(event_path, os.path.abspath(self.site.config['OUTPUT_FOLDER'])).replace(os.sep, '/')