Add null check on user path in mounts (#5446)

Author: mdegat01

supervisor/api/mounts.py

@@ -35,7 +35,9 @@ async def info(self, request: web.Request) -> dict[str, Any]:
                 mount.to_dict()
                 | {
                     ATTR_STATE: mount.state,
-                    ATTR_USER_PATH: mount.container_where.as_posix(),
+                    ATTR_USER_PATH: mount.container_where.as_posix()
+                    if mount.container_where
+                    else None,
                 }
                 for mount in self.sys_mounts.mounts
             ],