When there is no collision with a floor the get_floor_normal() function
should return the zero vector to be consistent with get_floor_velocity().

Renames floor_normal to up_direction in all bindings.

Updates the documentation of get_floor_normal() and get_floor_velocity()
to make it clear when the values are valid. Updates the documentation for
move_and_slide() and move_and_slide_with_snap() to use the new up_direction
parameter name.