Polishing

See spring-projectsgh-23649

spring-messaging/src/main/java/org/springframework/messaging/rsocket/DefaultRSocketRequester.java

@@ -117,7 +117,7 @@ private class DefaultRequestSpec implements RequestSpec {
 		private final MetadataEncoder metadataEncoder;
 
 		@Nullable
-		private Mono<Payload> payloadMono = Mono.empty();
+		private Mono<Payload> payloadMono = emptyPayload();
 
 		@Nullable
 		private Flux<Payload> payloadFlux = null;