Cache loader - remove legacy handling, handle null result from setting

Cache loader - remove legacy handling, handle null result from setting

We are seeing this symptom in our tests in some cases
https://civicrm.stackexchange.com/questions/28897/error-when-merging-accounts

I'm not quite sure why the setting would be empty - but treating empty the same as 'default'
seems better than a hard fail and we know that settings have a track history of not all being
loaded at the right times.

This also removes some legacy handling