server: allow NoneType for simln activity

src/warnet/server.py

@@ -270,7 +270,7 @@ def tank_messages(self, network: str, node_a: int, node_b: int) -> str:
             self.logger.error(msg)
             raise ServerError(message=msg) from e
 
-    def network_export(self, network: str, activity: str) -> bool:
+    def network_export(self, network: str, activity: str | None) -> bool:
         """
         Export all data for a simln container running on the network
         """