Only check for indexes on models with tables

Update the `LolDba::IndexFinding.model_classes` method to return only classes that have a table in the database.

lib/lol_dba/index_finding/index_finder.rb

@@ -68,7 +68,7 @@ def self.primary_key(table_name)
 
     def self.model_classes
       ActiveRecord::Base.descendants.select do |obj|
-        Class == obj.class && session_store?(obj)
+        Class == obj.class && session_store?(obj) && obj.table_exists?
       end
     end