Fix Multistore FF check for empty test DB

What does this MR do and why?

Fixes #346836 (closed) by safeguarding Feature Flag checks with feature_flags_available?.

How to set up and validate locally

RAILS_ENV=test bundle exec rake db:purge
RAILS_ENV=test bundle exec rake db:setup

will break on master, won't on this branch.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #346836 (closed)