Refine Reactor Netty handling of request without body

Closes gh-34003

spring-web/src/main/java/org/springframework/http/client/ReactorClientHttpRequest.java

@@ -145,7 +145,9 @@ private Publisher<Void> send(
 		headers.forEach((key, value) -> request.requestHeaders().set(key, value));
 
 		if (body == null) {
-			return outbound;
+			// NettyOutbound#subscribe calls then() and that expects a body
+			// Use empty Mono instead for a more optimal send
+			return Mono.empty();
 		}
 
 		AtomicReference<Executor> executorRef = new AtomicReference<>();

spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpRequest.java

@@ -120,7 +120,9 @@ public Mono<Void> writeWith(Path file, long position, long count) {
 
 	@Override
 	public Mono<Void> setComplete() {
-		return doCommit(this.outbound::then);
+		// NettyOutbound#then() expects a body
+		// Use null as the write action for a more optimal send
+		return doCommit(null);
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/http/client/reactive/ReactorNetty2ClientHttpRequest.java

@@ -123,7 +123,9 @@ public Mono<Void> writeWith(Path file, long position, long count) {
 
 	@Override
 	public Mono<Void> setComplete() {
-		return doCommit(this.outbound::then);
+		// NettyOutbound#then() expects a body
+		// Use null as the write action for a more optimal send
+		return doCommit(null);
 	}
 
 	@Override