Fix ConfigManagingActor crash if any path in config_paths doesn't exist

`event.path.samefile(p)` raises error if:
* event.path doesn't exists
* path `p` doesn't exists
This PR checks if `p` exists before calling this method.

Signed-off-by: Elzbieta Kotulska <[email protected]>

Author: ela-kotulska-frequenz

src/frequenz/sdk/config/_config_managing.py

@@ -176,7 +176,9 @@ async def _run(self) -> None:
                 # Since we are watching the whole parent directories, we need to make
                 # sure we only react to events related to the configuration files we
                 # are interested in.
-                if not any(event.path.samefile(p) for p in self._config_paths):
+                if not any(
+                    event.path.samefile(p) for p in self._config_paths if p.exists()
+                ):
                     continue
 
                 match event.type: