`google.protobuf.Empty` request failure

Hello ! This may be related to #702

I have the following proto:
```bash
syntax = "proto3";
import "google/protobuf/empty.proto";

service MoodService {
rpc checkMood(google.protobuf.Empty) returns (stream MoodResp);
}

message MoodResp {
enum Mood {
HAPPY = 0;
SAD = 1;
}
Mood mood = 1;
string desc = 2;
}
```
And the Javascript client code:
```javascript
const { MoodServiceClient } = require("./mood_grpc_web_pb.js");

var client = new MoodServiceClient(
"http://localhost:8080",
null,
null
);
```

When I'm trying to run this, I'm getting the following error:
```bash
return request.serializeBinary();
^
TypeError: request.serializeBinary is not a function
```

Setup:
Ubuntu 18.04.5
Node v14.14.0
Npm v6.14.8