That is *exactly* what I had to work with when I novelized The Butterfly Effect!
I was amazed to hear this, so I went back and checked my (admittedly unread) copy of the Terminator Salvation novelization, and not only does it not have the movie ending, it also directs you to buy the prequel From The Ashes "for the full story", which is a bit much.
Titan Books have done the "prequel novel" thing a few times in the past. As well as Terminator Salvation, they went through a phase of putting out prequels-then-novelizations with Warcraft, Pacific Rim Uprising, Independence Day Resurgance, War for the Planet of the Apes, Alien Covenant, The Predator, Battle Angel Alita and Noah.
I suppose in a way, that allows movie marketing have it's cake and eat it: release a prequel story before the film comes out to cash in on the hype but without spoiling the plotline, then release the actual novelization when the film hits...