Fix: Handle undefined values when resolving Fn::Join in environment variables

[connorads]

Fixes an edge case for #1032 which would cause a crash when environment variables have a value of undefined.

I can just set the environment variable value and then everything is fine but this error should probably still be fixed because it causes serverless-offline to crash where it need not.

But why would you have undefined environment variable value anyway?

Good question! Normally you shouldn't, because if you don't need an environment variable set you should probably remove it right?

I was developing locally and I didn't need certain environment variables set for local development with sls offline, but I did need them at deploy time. And it was working before I upgraded serverless-offline to v6.5.0 (for another feature).

Example: In my app, I only need the Slack Client ID when I deploy. But I still want to use sls offline to test parts of my app locally.

SLACK_CLIENT_ID: ${env:SLACK_CLIENT_ID}