[Ruby] Encoding JSON produces invalid JSON with empty StringValue wrapper.

[mintyfresh]

What version of protobuf and what language are you using?
Version: google-protobuf (3.9.2)
Language: Ruby

What operating system (Linux, Windows, ...) and version?
Ubuntu 18.04.2 LTS inside of WSL 2 (Windows Built 19018)

What runtime / compiler are you using (e.g., python version or gcc version)
ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-linux]

What did you do?
Steps to reproduce the behavior:

1. Define message containing StringValue field (defined by wrappers.proto).
2. Construct message object with StringValue wrapper containing empty String.
3. Encode message into JSON.

Example message type:

import "google/protobuf/wrappers.proto";

message User {
  google.protobuf.StringValue email = 1;
}

Example serialization:

user = User.new(email: Google::Protobuf::StringValue.new(value: ''))

User.encode_json(user)

What did you expect to see
Valid JSON string with empty key omitted:

{}

Or key containing empty wrapper object:

{"email": {}}

What did you see instead?
Invalid JSON string containing key with no value:

{"email":}

[sandlerben]

Also have this issue. Could you add a ruby tag to this issue?