Allow contributors to run docs site without env vars

[JohnTParsons]

Allow contributors to run docs site without env vars.

While working on CONTRIBUTING.md (here is the draft PR: #844), I noticed that README.md says, "> A few pages on the site use Google Sheets as a simple CMS, if you are contributing to the product you can request these variables from the team." Wouldn't be simpler and more secure if the docs site continued to work (as much as possible) even when no .env file containing secrets was present? Which was how it worked in early Jan.
Giving contributors access to LastPass doesn't seem like a good idea.

I suggest that we just return empty array IF env vars are not defined, instead of throwing CMSError. Only throw CMSError if CMS credentials don't work or they do work but data comes back in an unexpected format or missing some fields.

We could move checking that the env vars are set to the Circle CI step that does the docs site deployment. If not set, throw an error, instead of deploying, to make the pipeline fail.