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

fix.