Welcome Johann Sebastian Schicho as #curl commit author 1265: https://github.com/curl/curl/pull/13477
sendf: Curl_cwriter_write: remove comment disallowing zero length by schicho · Pull Request #13477 · curl/curl
Curl_client_write calls Curl_cwriter_write, which already has this limitation in place in its comment. blen is not checked in Curl_client_write. Stumbled upon this working on my other MQTT PR.GitHub