Don't throw errors if no player is set up to control.

if there's no `this.controls.target()`, `game.playerPosition()` returns `false`, yet all the methods that use `game.playerPosition()` ignore this possibility, thus, if you don't set up an object to control, the engine becomes unhappy. 

This gives the engine a sensible default in lieu of a real player position (the `game.camera.position`).