docs: def vector_unit_nullrand(v, rng=None):
      """Return unit vectors.
      Any null vectors are mapped to a uniformly picked unit vector.
  
      Parameters
      ----------
      v: array, shape (a1, a2, ..., d)
          Cartesian vectors, with last axis indexing the dimension.
  
      Returns
      -------
      v_new: array, shape of v
      """
      if v.size == 0:
          return v
      mag = vector_mag(v)
      v_new = v.copy()
      v_new[mag == 0.0] = sphere_pick(v.shape[-1], (mag == 0.0).sum(), rng)
      v_new[mag > 0.0] /= mag[mag > 0.0][..., np.newaxis]
      return v_new