Null RPC responses throw JSON parse error

[ChrisBates]

In initDirectReplyQueue() (connection.js) it's trying to parse what it expects to be JSON content inside the Buffer.

JSON.parse(msg.content.toString()),

If the Buffer is empty (i.e. the RPC handler returned a null/void/undefined) then it's trying to parse an empty string. This throws an error.

(node:2736) UnhandledPromiseRejectionWarning: SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at channel.consume.noAck (/usr/local/src/packages/wilco-svc-omniapi/node_modules/@golevelup/nestjs-rabbitmq/lib/amqp/connection.js:95:67)
    at ConfirmChannel.BaseChannel.dispatchMessage (/usr/local/src/packages/wilco-svc-omniapi/node_modules/amqplib/lib/channel.js:484:12)
    at ConfirmChannel.BaseChannel.handleDelivery (/usr/local/src/packages/wilco-svc-omniapi/node_modules/amqplib/lib/channel.js:493:15)
    at ConfirmChannel.emit (events.js:314:20)
    at /usr/local/src/packages/wilco-svc-omniapi/node_modules/amqplib/lib/channel.js:279:10
    at ConfirmChannel.headers (/usr/local/src/packages/wilco-svc-omniapi/node_modules/amqplib/lib/channel.js:306:9)
    at ConfirmChannel.C.acceptMessageFrame (/usr/local/src/packages/wilco-svc-omniapi/node_modules/amqplib/lib/channel.js:247:31)
    at ConfirmChannel.C.accept (/usr/local/src/packages/wilco-svc-omniapi/node_modules/amqplib/lib/channel.js:400:17)
    at Connection.mainAccept [as accept] (/usr/local/src/packages/wilco-svc-omniapi/node_modules/amqplib/lib/connection.js:64:33)
    at Socket.go (/usr/local/src/packages/wilco-svc-omniapi/node_modules/amqplib/lib/connection.js:478:48)
    at Socket.emit (events.js:314:20)
    at emitReadable_ (_stream_readable.js:557:12)
    at processTicksAndRejections (internal/process/task_queues.js:83:21)

I was going to propose handling it the same way it's currently being handled in handleMessage() with a try/catch, this seems robust but would also resolve non-JSON content as undefined (not just null values) which may be undesirable.

Instead I propose more specifically handling an empty Buffer to prevent JSON parse from throwing the error. Will submit a PR.

[WonderPanda]

@ChrisBates Thanks for the contribution. This is available as of @golevelup/nestjs-rabbitmq@1.17.1