Fix find_nth with limit_value

[kamipo]

If the index exceeds a limit, simply return an empty result without
querying the database.

[rails-bot]

r? @senny

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[kamipo]

@pixeltrix Could you review the fix?

[pixeltrix]

@kamipo I was a little concerned that relation.limit(limit).to_a could result in loading large result sets but I don't think there was anyway it could be nil - took a while to check the various code paths though.

Thanks for fixing 👍