Fix configuring search when search settings are empty.
Review Request #11427 — Created Feb. 2, 2021 and submitted
The recent work to fix search settings and bring compatibility to modern
Haystack ended up regressing configuring a search engine if there
weren't already stored settings. The old code conditionally ensured,
rather than assumed, there would be a key in settings for the search
backend, but the new code regressed that.
This change updates the search configuration setting code to always set
a default entry if it doesn't exist, and to populate that.
Unit tests were added to ensure this doesn't regress again.
Unit tests pass.
Verified the original failure case with the unit tests, without the