Encoding a message with a null string field throws an error

[paralin]

protobuf.js version:

├─ protobufjs@6.6.5
│  ├─ @protobufjs/aspromise@^1.1.1
│  ├─ @protobufjs/base64@^1.1.1
│  ├─ @protobufjs/codegen@^1.0.8
│  ├─ @protobufjs/eventemitter@^1.1.0
│  ├─ @protobufjs/fetch@^1.1.0
│  ├─ @protobufjs/inquire@^1.1.0
│  ├─ @protobufjs/path@^1.1.2
│  ├─ @protobufjs/pool@^1.1.0
│  ├─ @protobufjs/utf8@^1.1.0
│  ├─ @types/long@^3.0.31
│  ├─ @types/node@7.0.5
│  └─ long@^3.2.0

Encoding a message with string fields filled in with null seems to cause the following error in Node:

TypeError: "string" must be a string, Buffer, or ArrayBuffer
    at Function.byteLength (buffer.js:362:11)
    at BufferWriter.write_string_buffer [as string] (/home/paralin/Documents/goworkspace/src/github.com/fuserobotics/fusecloud-common/node_modules/protobufjs/src/writer_buffer.js:68:22)
    at Type._rgraphql_RGQLQueryTreeNode$encode [as encode] (eval at eof (/home/paralin/Documents/goworkspace/src/github.com/fuserobotics/fusecloud-common/node_modules/@protobufjs/codegen/index.js:103:25), <anonymous>:9:16)
    at Type._rgraphql_RGQLSerialOperation$encode [as encode] (eval at eof (/home/paralin/Documents/goworkspace/src/github.com/fuserobotics/fusecloud-common/node_modules/@protobufjs/codegen/index.js:103:25), <anonymous>:15:12)
    at Type._rgraphql_RGQLClientMessage$encode [as encode] (eval at eof (/home/paralin/Documents/goworkspace/src/github.com/fuserobotics/fusecloud-common/node_modules/@protobufjs/codegen/index.js:103:25), <anonymous>:9:12)
    at Type._socketbus_SBClientMessage$encode [as encode] (eval at eof (/home/paralin/Documents/goworkspace/src/github.com/fuserobotics/fusecloud-common/node_modules/@protobufjs/codegen/index.js:103:25), <anonymous>:9:12)
    at repl:1:24
    at realRunInThisContextScript (vm.js:22:35)
    at sigintHandlersWrap (vm.js:98:12)
    at ContextifyScript.Script.runInThisContext (vm.js:24:12)

And the following in the browser:

TypeError: Cannot read property 'length' of null
    at Object.utf8_length [as length] (index.js:18)
    at Writer.write_string [as string] (writer.js:491)
    at Type._rgraphql_RGQLQueryTreeNode$encode [as encode] (eval at eof (index.js:103), <anonymous>:9:16)
    at Type.encode_setup [as encode] (type.js:406)
    at Type._rgraphql_RGQLSerialOperation$encode [as encode] (eval at eof (index.js:103), <anonymous>:15:12)
    at Type.encode_setup [as encode] (type.js:406)
    at Type._rgraphql_RGQLClientMessage$encode [as encode] (eval at eof (index.js:103), <anonymous>:9:12)
    at Type.encode_setup [as encode] (type.js:406)
    at Type._socketbus_SBClientMessage$encode [as encode] (eval at eof (index.js:103), <anonymous>:9:12)

Null strings should be treated as empty value, IMO. I haven't had a chance to make an isolated repro yet. Will post one tomorrow if you haven't already fixed this.

[dcodeIO]

This is true for primitive types, like string, number and boolean.

Relevant line is here: https://github.com/dcodeIO/protobuf.js/blob/master/src/encoder.js#L115

[dcodeIO]

Should be working in master.