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
b3a45670f9