[CIVIS-1972] BUG catch empty API response

[KasiaHinkson]

Wrapped response.json() in a try-except to catch JSONDecodeErrors in CivisAPIError.

• [x] (For Civis employees only) Reference to a relevant ticket in the pull request title
• [x] Changelog entry added to CHANGELOG.md at the repo's root level
• [x] Description of change in the pull request description
• [x] If applicable, unit tests have been added and/or updated
• [x] The CircleCI builds have all passed