Fix JSON printing of Any of an empty message in C++Proto.

This check was likely based on confusion about presence: if the 'bytes' field of Any supported presence and was unset it would be a malformed message.

However, as a Proto3 message the bytes field is implicit presence, and an empty message serializes to 0-bytes. It is perfectly valid to pack an empty message into an Any, and there's no distinguishing between "they forgot to set the value" and "actual zero-length serialization".

This wasn't caught by a user sooner since it is already allowed when allow_legacy_nonconformant_behavior is on, and that setting is on by default and rarely disabled.

#24937

PiperOrigin-RevId: 848253357

Author: protobuf-github-bot

src/google/protobuf/json/internal/unparser.cc

@@ -745,9 +745,6 @@ absl::Status WriteAny(JsonWriter& writer, const Msg<Traits>& msg,
     return absl::OkStatus();
   } else if (!has_type_url) {
     return absl::InvalidArgumentError("broken Any: missing type URL");
-  } else if (!has_value &&
-             !writer.options().allow_legacy_nonconformant_behavior) {
-    return absl::InvalidArgumentError("broken Any: missing value");
   }
 
   writer.Write("{");

src/google/protobuf/json/json_test.cc

@@ -1179,6 +1179,23 @@ TEST_P(JsonTest, TestOverwriteRepeated) {
 }
 
 
+TEST_P(JsonTest, TestAny) {
+  // Not setting 'value' is legal because it is the the representation of an
+  // empty message (since 'bytes' is an implicit presence field).
+  google::protobuf::Any any;
+  any.set_type_url("type.googleapis.com/proto3.TestMessage");
+  EXPECT_THAT(
+      ToJson(any),
+      IsOkAndHolds(R"({"@type":"type.googleapis.com/proto3.TestMessage"})"));
+  EXPECT_THAT(
+      ToJson(any, {.allow_legacy_nonconformant_behavior = false}),
+      IsOkAndHolds(R"({"@type":"type.googleapis.com/proto3.TestMessage"})"));
+
+  auto round_trip = ToProto<google::protobuf::Any>(*ToJson(any));
+  ASSERT_OK(round_trip);
+  EXPECT_THAT(any, testing::EqualsProto(*round_trip));
+}
+
 TEST_P(JsonTest, TestDuration) {
   auto m = ToProto<proto3::TestDuration>(R"json(
     {