roachpb: Protobuf encoding does not omit empty messages

[erikgrinaker]

As seen over in #85138 (comment), it appears that gogoproto is encoding embedded messages even though they are empty. In other words, consider this message:

message Foo {
  Timestamp timestamp = 1; 
}

message Timestamp {
  int64 wall_time = 1;
  int32 logical = 2;
  bool synthetic = 3;
}

Even though empty Protobuf fields can be omitted, gogoproto will encode an empty Foo into 2 bytes: 0x0a 0x00, which essentially says field 1 of wire type delimited with length 0 (see encoding spec).

The encoding should be able to omit this field entirely, which might net us some minor savings throughout the codebase (e.g. for every unset timestamp field).

Jira issue: CRDB-18178

[erikgrinaker]

@nvanbenschoten points out that this may only happen for non-nullable fields, and refers to ResponseHeader.MarshalToSizedBuffer and it's handling of the Txn field.

[erikgrinaker]

Yeah, this is a gogoproto-ism.