proto: add explicit test for a non-nil but empty byte slice

I encountered this usage pattern in a production application
and noticed that when breaking the size calculation,
only the conformance_test fails (indirectly),
but there was no explicit test before this commit.

Change-Id: I15a125b7cff3364f24fe372e4a3a046bb7552413
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/710855
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: stapelberg

proto/size_test.go

@@ -5,7 +5,12 @@
 package proto_test
 
 import (
+	"testing"
+
+	"google.golang.org/protobuf/encoding/prototext"
+	"google.golang.org/protobuf/encoding/protowire"
 	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/types/known/anypb"
 )
 
 // Checking if [Size] returns 0 is an easy way to recognize empty messages:
@@ -16,3 +21,15 @@ func ExampleSize() {
 		// skip processing this message, or return an error, or similar.
 	}
 }
+
+func TestSizeAnyNonNilButEmpty(t *testing.T) {
+	ne := &anypb.Any{
+		TypeUrl: "abc",
+		Value:   []byte{},
+	}
+
+	want := protowire.SizeBytes(len("abc")) + protowire.SizeTag(1 /* TypeUrl */)
+	if got := proto.Size(ne); got != want {
+		t.Errorf("proto.Size(%v) = %v, want %v", prototext.Format(ne), got, want)
+	}
+}