You can cast Gravepurge with no targets. If you do, you’ll just draw a card. However, if you cast it with at least one target and all of those targets become illegal, the spell won’t resolve and you won’t draw a card.

You choose the order of the cards you put on top of your library.

You can cast Gravepurge with zero targets. If you do, you’ll draw a card as the spell resolves.

If you cast Gravepurge with at least one target and all of those targets become illegal, the spell won’t resolve and you won’t get to draw a card.