Prior to this commit, `BufferingClientHttpRequestWrapper` would always
write to the actual client request body, even if the buffered content
was empty (empty byte array).
This would cause issues with specific client request factories,
especially the OkHttp variant, that would consider empty byte arrays as
non-empty body and would reject such cases for GET requests with an
"IllegalArgumentException: method GET must not have a request body".
This commit only writes to the request if the buffered content is not
empty.
Fixes gh-32612
019ce4418c