Fixed problem with through association with STI. #14046

There were a problem with through association with STI  #14046

The problem was caused by `Foo.unscoped`. If model `Foo` was create via STI `Foo.unscoped.valuse[:where]` returns where condition to handle the STI logic.

But on through association on the first place we don't want to put his were clause to join table. I simply added check for `reflection.scope`. And if it is blank than simply skip this.