Don't attempt to load SettingsChange too early

This is an attempt to reimplement ManageIQ#22853 since that was
reverted in ManageIQ#23038.

lib/vmdb/settings/database_source.rb

@@ -75,7 +75,7 @@ def my_server
       end
 
       def resource_queryable?
-        ActiveRecord::Base.connectable? && ::SettingsChange.table_exists?
+        ActiveRecord::Base.connectable? && defined?(::SettingsChange) && ::SettingsChange.table_exists?
       end
     end
   end