pw_protobuf: Break the ambiguity for spans in generated code
pw_protobuf: Break the ambiguity for spans in generated code

Some std::span implementations has an ambiguity in the resolving
(ptr, 0) span construction. This CL adds explicit size_t for the second
arg to avoid it.

Change-Id: Ib2e0c3319a5841f64981d7f4c5d62b5d3dea8ebd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/99285
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Pigweed-Auto-Submit: Dennis Kormalev <6222e5e705620133a24041cba003e993480020a9@google.com>

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