Fix generated code compatibility with strict null checks

[yunyu]

For this proto file:

syntax = "proto3";

message JoinMessage { string channel = 1; }

message ChatMessage {
  uint32 id = 1;
  string message = 2;
}

message RPCMessage {
  oneof msg {
    JoinMessage join = 1;
    ChatMessage chat = 2;
  }
}

This change generates:

// code generated by pbf v3.2.0
/* tslint:disable */

import Pbf from 'pbf';

export interface IJoinMessage {
    channel?: string;
}

export interface IChatMessage {
    id?: number;
    message?: string;
}

export interface IRPCMessage {
    msg?: "join" | "chat";
    join?: IJoinMessage;
    chat?: IChatMessage;
}

export const JoinMessage = {
    read(pbf: Pbf, end?: number): IJoinMessage {
        return pbf.readFields(JoinMessage._readField, {channel: ""}, end);
    },
    _readField(tag: number, obj: any, pbf?: Pbf): void {
        if (tag === 1 && pbf) obj.channel = pbf.readString();
    },
    write(obj: IJoinMessage, pbf?: Pbf): void {
        if (obj.channel && pbf) pbf.writeStringField(1, obj.channel);
    }
};

export const ChatMessage = {
    read(pbf: Pbf, end?: number): IChatMessage {
        return pbf.readFields(ChatMessage._readField, {id: 0, message: ""}, end);
    },
    _readField(tag: number, obj: any, pbf?: Pbf): void {
        if (tag === 1 && pbf) obj.id = pbf.readVarint();
        else if (tag === 2 && pbf) obj.message = pbf.readString();
    },
    write(obj: IChatMessage, pbf?: Pbf): void {
        if (obj.id && pbf) pbf.writeVarintField(1, obj.id);
        if (obj.message && pbf) pbf.writeStringField(2, obj.message);
    }
};

export const RPCMessage = {
    read(pbf: Pbf, end?: number): IRPCMessage {
        return pbf.readFields(RPCMessage._readField, {join: undefined, msg: undefined, chat: undefined}, end);
    },
    _readField(tag: number, obj: any, pbf?: Pbf): void {
        if (tag === 1 && pbf) obj.join = JoinMessage.read(pbf, pbf.readVarint() + pbf.pos), obj.msg = "join";
        else if (tag === 2 && pbf) obj.chat = ChatMessage.read(pbf, pbf.readVarint() + pbf.pos), obj.msg = "chat";
    },
    write(obj: IRPCMessage, pbf?: Pbf): void {
        if (obj.join && pbf) pbf.writeMessage(1, JoinMessage.write, obj.join);
        if (obj.chat && pbf) pbf.writeMessage(2, ChatMessage.write, obj.chat);
    }
};

Note the optional pbf argument and if check. This change makes the generated files compatible with @types/pbf with --strictNullChecks.

[yunyu]

It looks like the issue that this fixes was discussed higher up in the thread:

mapbox#107 (comment)

Your call, it might not be worth merging this workaround and PRing a fix to @types/pbf.