TrackBallControls: allow mouse control to be null

[AyaHelmy9]

When using TypeScript the mouse control is not allowed to be nullable when in fact I wish to not have any control for a particular mouse button.
Currently I have to do this to make it work:

mouseButtons={{
          LEFT: three.MOUSE.RIGHT, // map left behaviour (rotate) to right mouse click
          MIDDLE: null as unknown as three.MOUSE, // map middle behaviour (zoom) to nothing
          RIGHT: three.MOUSE.MIDDLE, // map right behaviour (pan) to middle mouse click
        }}

Note that this is similar to #144 but this time for TrackBallControls.