Fix variable mismatch in code quality widget

What does this MR do?

Fix a bug (https://gitlab.com/gitlab-org/gitlab-ee/issues/13026) generated by https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14323

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14323 moved the code quality comparison calculation into a worker thread, and passed data to the worker thread to for it to be processed and then passed back, but the parameters being passed got out of sync: the worker was expecting data.parsedHeadIssues and data.parsedBaseIssues but was passed data.headIssues and data.baseIssues instead.

This MR

• fixes the worker to use data.headIssues and data.baseIssues
• makes the worker send back an empty object if its input data does not include headIssues and baseIssues
• makes the store that calls the worker report an error loading/parsing the code quality results if the worker does not pass back data.newIssues and data.resolvedIssues

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers