Allow null response

[pro-wh]

We're using a custom codec, where null or undefined responses can be represented, and we use that for void methods, similar to how protobuf uses Empty.

This change allows grpc-web to accept responses that decode to null or undefined.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ pro-wh (a048c07)

[pro-wh]

I couldn't build this locally:

$ npm run build

> [email protected] build /home/wh0/work/ekiden/grpc-web/packages/grpc-web
> node scripts/build.js

/home/wh0/work/ekiden/grpc-web/packages/grpc-web/node_modules/.bin/google-closure-compiler --js=exports.js --js=../../javascript --js=../../third_party/closure-library --entry_point=grpc.web.Exports --externs=externs.js --dependency_mode=PRUNE --compilation_level=ADVANCED_OPTIMIZATIONS --generate_exports --export_local_property_definitions --js_output_file=index.js
../../javascript/net/grpc/web/clientreadablestream.js:29: ERROR - [JSC_UNDEFINED_VARIABLE] variable goog is undeclared
goog.module('grpc.web.ClientReadableStream');
^^^^

1 error(s), 0 warning(s)

[pro-wh]

EasyCLA won't accept my CLA for this. It only seems to apply it to another PR I opened, #1026

[pro-wh]

they said I should close and reopen to get the bot to notice the CLA. doing that now

[pro-wh]

this turned out not to solve the problem. there are no data events for these payloads at all. opening #1230 as a proposal to add data events for falsy responses