You have to follow your instincts, depending on the type of people you work for there are several different ways this can go.
Some will spit in the face of your two weeks' notice and ask you to leave immediately, which is kind of a childish reaction to having you upset their plans.
Some will try to get you to commit to just one more week at a time until they've extracted what they need, and work you harder during that time.
Some will accept that you're not happy, can't do the best work because you aren't happy, and will work with you to tie up loose ends with your 2 weeks notice, and maybe even let you off a little sooner if, as another commenter suggested, you ask for that contingency.
Unfortunately, it's not clear cut. If you have reason to be confident that one of the first two is going to happen, then you shouldn't give notice. In the latter case, burning a bridge would be a shame since you aren't giving them a chance to be considerate.
The last time I worked a contract gig, the manager essentially dragged a guy out back and shot him on my third day. That undermined anything they did to make me feel like there might be a future, or that a place on their team might be worthwhile. I received full-time offers every week that I worked for them, including the first one, and when I finally chose one, set a start date two weeks out, and notified my manager, he walked me out the door, and I had to shuffle my plans a bit.
If you don't have something new lined up yet, it's probably best to find a way to give notice, though as many others have said, you shouldn't feel bad about leaving their project understaffed - it sounds like it already is.
Some will spit in the face of your two weeks' notice and ask you to leave immediately, which is kind of a childish reaction to having you upset their plans.
Some will try to get you to commit to just one more week at a time until they've extracted what they need, and work you harder during that time.
Some will accept that you're not happy, can't do the best work because you aren't happy, and will work with you to tie up loose ends with your 2 weeks notice, and maybe even let you off a little sooner if, as another commenter suggested, you ask for that contingency.
Unfortunately, it's not clear cut. If you have reason to be confident that one of the first two is going to happen, then you shouldn't give notice. In the latter case, burning a bridge would be a shame since you aren't giving them a chance to be considerate.
The last time I worked a contract gig, the manager essentially dragged a guy out back and shot him on my third day. That undermined anything they did to make me feel like there might be a future, or that a place on their team might be worthwhile. I received full-time offers every week that I worked for them, including the first one, and when I finally chose one, set a start date two weeks out, and notified my manager, he walked me out the door, and I had to shuffle my plans a bit.
If you don't have something new lined up yet, it's probably best to find a way to give notice, though as many others have said, you shouldn't feel bad about leaving their project understaffed - it sounds like it already is.