A spacelike subspace of a Lorentz vector space is one for which the restricted Lorentz scalar product is positive-definite. The restriction of the Lorentz scalar product to the zero subspace is technically positive-definite, because you cannot find a non-zero vector there (!) whose scalar square is non-positive.
A spacelike subspace of a Lorentz vector space is one for which the restricted Lorentz scalar product is positive-definite. The restriction of the Lorentz scalar product to the zero subspace is technically positive-definite, because you cannot find a non-zero vector there (!) whose scalar square is non-positive.