[roll third_party/pigweed] pw_transfer: Fix integration test START packet issue

The proxy in pw_transfer's integration tests were receiving multiple
START packets, resulting in incorrect behavior in tests. This occurred
due to an assumption that all RPC packets contained transfer chunks and
attempting to decode them. An empty RPC payload resulted in a default
(zero-valued) chunk, which was interpreted as a START chunk for legacy
protocol compatibility.

This fixes the issue by only considering RPC packets which actually
have a payload.

Original-Bug: 322497491
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/192755

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 36b6a7ddb0c28b6..b83c8ed0ce86ff9
Roller-URL: https://ci.chromium.org/b/8755524075590442049
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iea7b5c748366e6bb485524bf0f5c5b5c565442d5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/193322
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 36b6a7d..b83c8ed 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 36b6a7ddb0c28b66a6d3420b99104a5869f37363
+Subproject commit b83c8ed0ce86ff90a09437ea719943f366e8c78f