fix https://linear.app/tryghost/issue/SLO-104/cannot-read-properties-of-undefined-reading-0-an-unexpected-error
- if the request body didn't contain the correct keys, it'd just HTTP
500 out of there
- this adds some optional chaining so we end up with undefined if
anything isn't as expected, and the following if-statement does the
rest of the check for us
- this also adds a breaking test (the first E2E test for authentication, yay!)
