encoding/protojson: use fd.HasPresence() for editions compatibility

This change was originally done by Mike Kruskal in Google-internal cl/515091827.

Change-Id: I5d1588193663249964cf8bc9b9fe9f61d29c98a6
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/606756
Reviewed-by: Christian Höppner <hoeppi@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

encoding/protojson/encode.go

@@ -216,9 +216,7 @@ func (m unpopulatedFieldRanger) Range(f func(protoreflect.FieldDescriptor, proto
 		}
 
 		v := m.Get(fd)
-		isProto2Scalar := fd.Syntax() == protoreflect.Proto2 && fd.Default().IsValid()
-		isSingularMessage := fd.Cardinality() != protoreflect.Repeated && fd.Message() != nil
-		if isProto2Scalar || isSingularMessage {
+		if fd.HasPresence() {
 			if m.skipNull {
 				continue
 			}