http: adding doc and debug for calling empty string on write function

PR-URL: #22118
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

doc/api/http.md

@@ -750,6 +750,9 @@ Returns `true` if the entire data was flushed successfully to the kernel
 buffer. Returns `false` if all or part of the data was queued in user memory.
 `'drain'` will be emitted when the buffer is free again.
 
+When `write` function is called with empty string or buffer, it does
+nothing and waits for more input.
+
 ## Class: http.Server
 <!-- YAML
 added: v0.1.17

lib/_http_outgoing.js

@@ -599,7 +599,10 @@ function write_(msg, chunk, encoding, callback, fromEnd) {
 
   // If we get an empty string or buffer, then just do nothing, and
   // signal the user to keep writing.
-  if (chunk.length === 0) return true;
+  if (chunk.length === 0) {
+    debug('received empty string or buffer and waiting for more input');
+    return true;
+  }
 
   if (!fromEnd && msg.connection && !msg[kIsCorked]) {
     msg.connection.cork();