Add test

Fixed typo.

- Using 'this vector' to describe the vector on which the method
performs actions (self).

- Using 'the vector' to describe a vector that is returned (from linear
interpolation between 2 vectors).

- Using 'a vector' to describe a vector in the arguments of a method, or
a hypothetical vector.

Only default radius to -1, makeEmpty, and sphere that returns isEmpty will also produce an empty box3

Added new empty concept to sphere with a negative radius. Modified containment / intersection functions to honor isEmpty and return false in this case