test better websockets test fixture

Author: jell-o-fishi

tests/tools/fixtures_websockets.py

@@ -40,9 +40,10 @@ async def server_app():
 
     finally:
         stop_websocket_server.set()
-        await server.close()
+        if server is not None:
+            await server.close()
 
-        assert_no_open_streams(client, server)
+            assert_no_open_streams(client, server)
 
         try:
             server_task.cancel()