Filling Gap Between External Door Frame and Wall - For Securing Strike Plate Screws
You could put some shims/wood filler/bondo in there, but it's 90% cosmetic. (The remaining 10% is that it would resist an attack with a prybar, but realistically, if someone is coming at your door with a prybar, you're doomed.)

The screws (assuming they're long enough to bite solidly into the framing), will do what they're supposed to do with or without filler.