align ping message with graphql-sse server implementation (https://github.com/enisdenjo/graphql-sse/blob/e8bef032422a7d38a670dc6d18204c4f5dfab6c8/src/handler.ts#L516)

Author: danplischke

ariadne/asgi/handlers/http.py

@@ -231,7 +231,7 @@ async def _ping(self, send: Send) -> None:
                 await send(
                     {
                         "type": "http.response.body",
-                        "body": self.encode_event(GraphQLServerSentEvent(event="next")),
+                        "body": ":\n\n".encode(self.encoding),
                         "more_body": True,
                     }
                 )