[roll third_party/pigweed] pw_protobuf: Don't write empty fixed sized fields

Fixed sized fields are an implementation detail of pw_protobuf, and
should behave identically on-wire to those implemented with pw::Vector.
This means not encoding when they have a zero (default) value, for
compatibility with other protobuf implementations.

Confirmation that this is correct behavior is that after this, tests
for certain fields no longer need to include expected zero values
of random other fields.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/97901

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 19f3a3d1077c6fc..9a41bfeb2355143
Roller-URL: https://ci.chromium.org/b/8811429182687464257
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8811429182687464257
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id160ea56ceb1f673f8ac8ab5ca7dae4105d62e61
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/97950
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 19f3a3d..9a41bfe 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 19f3a3d1077c6fcccbd527a5aced0e8c4c02f6ab
+Subproject commit 9a41bfeb2355143d3d2fb3d8f78a9b973ba809f7