Dynamically created message descriptors do not track field presence for proto3 optional fields

[ruiQQ]

Hi,

I am building a dynamic message from a dynamic file descriptor, and I noticed that the dynamic fds seem to ignore proto3 optional settings

Steps to reproduce:

1. Build FileDescriptorProto using FileDescriptor#new_dynamic_fds
2. Get MessageDescriptor and create a new instance of the message
3. Set a proto3 optional field to 0
4. Serialize message into bytes using MessageDyn.html#write_to_bytes_dyn, observed that 0 is not encoded, field presence is not tracked

In my FileDescriptorProto, I have this MessageDescriptorProto, where I set the proto3_optional to true, and added a synthetic oneof descriptor to make this field explicitly optional

    message_type: {
        name: "Foo",
        field: {
                label: LABEL_OPTIONAL,
                type: TYPE_INT32,
                name: "bar",
                number: 1
                proto3_optional: true,
                oneof_index: 0,
        },
        oneof_decl: {
            name: "bar",
        }
    },

Is there an error in my MessageDescriptor proto set up? How can I make the dynamic message from dynamic fds track field presence?

Thank you!