pw_transfer: Fix when to write Chunk::PendingBytes

Change-Id: Ie7ac64ef6ce5bfcade62fab93a7450e939a30727
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/98521
Commit-Queue: Carlos Chinchilla <cachinchilla@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
Pigweed-Auto-Submit: Carlos Chinchilla <cachinchilla@google.com>

pw_transfer/chunk.cc

@@ -207,7 +207,9 @@ Result<ConstByteSpan> Chunk::Encode(ByteSpan buffer) const {
 
     // In the legacy protocol, the pending_bytes field must be set alongside
     // window_end_offset, as some transfer implementations require it.
-    encoder.WritePendingBytes(window_end_offset_ - offset_).IgnoreError();
+    if (window_end_offset_ != 0) {
+      encoder.WritePendingBytes(window_end_offset_ - offset_).IgnoreError();
+    }
   }
 
   if (max_chunk_size_bytes_.has_value()) {