The application of "Send the top 2 cards of your Deck to the Graveyard" and the application of "then draw 1 card for each "Lightsworn" monster sent to the Graveyard by this effect" are not treated as being done at the same time.
If no "Lightsworn" monsters were sent to the Graveyard as a result of the application of "Send the top 2 cards of your Deck to the Graveyard", "then" is not applied. (It is treated as finishing with the application of "Send […] to the Graveyard".)
Previously Official Rulings
These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: 
These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official, but may or may not necessarily hold true nowadays.
If you do not have enough cards to send to the Graveyard remaining in your Deck when resolving the effect of a “Lightsworn” monster or “Judgment Dragon,” all cards remaining in your Deck are sent to the Graveyard. For example, if you control “Jain, Lightsworn Paladin” and its effect activates during your End Phase when you have only 1 card remaining in your Deck, that card will be sent to the Graveyard.