encoding/protojson: fix google.protobuf.Empty serialization

Go Protobuf was diverging from other Protobuf implementations.
Worse: The other Protobuf implementations cannot decode
what Go Protobuf encodes for a google.protobuf.Empty message.

By now, the spec has been updated to explicitly spell out
that google.protobuf.Empty is not considered special and
should result in {"@type": "type.googleapis.com/google.protobuf.Empty"}
without an extra `"value": {}`.

The fixed encoding is decoded correctly with Go Protobuf
v1.35.2 (released November 2024) or newer.

Fixes golang/protobuf#1707

Fixes golang/protobuf#1620

For protocolbuffers/protobuf#5390

For protocolbuffers/protobuf#17099

For protocolbuffers/protobuf#24445

Change-Id: I54f4d842e7272743191ec274b8e7434ddb74203f
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/735120
Reviewed-by: Lasse Folger <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cassondra Foesch <[email protected]>

Author: stapelberg

encoding/protojson/encode_test.go

@@ -1717,8 +1717,7 @@ func TestMarshal(t *testing.T) {
 			}
 		}(),
 		want: `{
-  "@type": "type.googleapis.com/google.protobuf.Empty",
-  "value": {}
+  "@type": "type.googleapis.com/google.protobuf.Empty"
 }`,
 	}, {
 		desc: "Any with StringValue containing invalid UTF8",
@@ -1927,8 +1926,7 @@ func TestMarshal(t *testing.T) {
   "optValue": "world",
   "optEmpty": {},
   "optAny": {
-    "@type": "google.protobuf.Empty",
-    "value": {}
+    "@type": "google.protobuf.Empty"
   },
   "optFieldmask": "fooBar,barFoo"
 }`,

encoding/protojson/well_known_types.go

@@ -52,7 +52,10 @@ func wellKnownTypeMarshaler(name protoreflect.FullName) marshalFunc {
 		case genid.FieldMask_message_name:
 			return encoder.marshalFieldMask
 		case genid.Empty_message_name:
-			return encoder.marshalEmpty
+			// The spec explicitly specifies that the Empty message
+			// is not considered to have any special JSON mapping:
+			// https://protobuf.dev/programming-guides/json/#any
+			return nil
 		}
 	}
 	return nil