Handle empty body

[anne23]

to avoid no implicit conversion of nil into String errors when using -w

Pull Request Checklist

There's no open issue that I can find for this, but it's fixing something that has confused people quite often in our check output.

General

• [x] Update Changelog following the conventions laid out here

• [x] RuboCop passes

• [x] Existing tests pass - rake integration passes, but I can't successfully run rake spec locally against master either - automated tests all passed on travis though

Purpose

to avoid no implicit conversion of nil into String errors when using -w and getting an empty body in the response, because this generally isn't a very helpful indication of what the actual problem is when a check starts failing and can get in the way of people investigating the real issue

Known Compatibility Issues

[jspaleta]

LGTM, merging fix should show up in next release