CoverageReportsController HTTP request malformated when ref_path is not provided

Instead of manually forming the URL we might be able to use setUrlParams or one of the other utils from url_utility.js. Failing that we should look around to see if we can avoid hand-rolling this path.

The following discussion from !50155 (merged) should be addressed:

• @iamricecake started a discussion: (+1 comment)

  This seems weird that we have to pass an empty string here for ref_path. It would make more sense to just not pass ref_path at all. So I feel like we may be doing something wrong on the frontend when building the request.

  I understand this is urgent though so let's address this on a follow-up.