Do not set 0 Content-Length header in BufferingClientHttpRequestFactory

Prior to this commit, the `BufferingClientHttpRequestFactory`, through
the `AbstractBufferingClientHttpRequest`, would set a "Content-Length"
header value, even if the buffered body was empty.

This behavior is invalid since no request body would be set by the
client code in the first place.

This commit ensures that this header is only set if a request body has
been buffered and is about to be written to the request.

Fixes gh-32650

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -42,7 +42,7 @@ abstract class AbstractBufferingClientHttpRequest extends AbstractClientHttpRequ
 	@Override
 	protected ClientHttpResponse executeInternal(HttpHeaders headers) throws IOException {
 		byte[] bytes = this.bufferedOutput.toByteArrayUnsafe();
-		if (headers.getContentLength() < 0) {
+		if (bytes.length > 0 && headers.getContentLength() < 0) {
 			headers.setContentLength(bytes.length);
 		}
 		ClientHttpResponse result = executeInternal(headers, bytes);