Prior to this commit, HTTP requests sent with the
`HttpComponentsClientHttpRequestFactory` would not set a
"Content-Length" header for empty request bodies. Setting a request
entity is the expected behavior for unsafe HTTP methods, and this would
align the behavior with other HTTP clients.
Developers would often rely on `BufferingClientHttpRequestFactory` to
set this information on the request.
This commit ensures that a `NullEntity` is used for unsafe HTTP methods,
when no body has been set for the request. This result in a
"Content-Length:0" request header.
Fixes gh-32678
47c5cd208c