fix: [ERR_MISSING_ARGS]: The "message" argument must be specified when using --useChildProcesses

[khanguyen88]

Description

When a function handler does not return anything in --useChildProcesses, the below error is raised

(node:88622) UnhandledPromiseRejectionWarning: TypeError [ERR_MISSING_ARGS]: The "message" argument must be specified
    at process.target._send (internal/child_process.js:693:13)
    at process.target.send (internal/child_process.js:678:19)
    at process.<anonymous> (/Users/khanguyen/git/atlas-authority/NAFJC/connect/node_modules/serverless-offline/dist/lambda/handler-runner/child-process-runner/childProcessHelper.js:48:11)
(node:88622) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:88622) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

fixes #1473

Motivation and Context

The child process won't be terminated properly and cause memory leak.

How Has This Been Tested?

I don't know if there is any existing unit test for this. I only did testing in my local project where my handler does not return anything, and with this change, the above error disappears.

Screenshots (if appropriate):

[dnalborczyk]

thank you for the PR @khanguyen88