Fix aiohttp SocketModeClient.is_connected AttributeError in detailed …

…logging

In a new aiohttp SocketModeClient, when self.current_session is None, calling is_connected and going into the detailed logging section will attempt to access self.current_session.closed and lead to an AttributeError

slack_sdk/socket_mode/aiohttp/__init__.py

@@ -332,7 +332,7 @@ async def is_connected(self) -> bool:
                 f"session_id: {session_id}, "
                 f"closed: {self.closed}, "
                 f"stale: {self.stale}, "
-                f"current_session.closed: {self.current_session.closed}, "
+                f"current_session.closed: {self.current_session and self.current_session.closed}, "
                 f"is_ping_pong_failing: {is_ping_pong_failing}"
                 ")"
             )