fix(lt): reject zero-vector output in Lt callable constructor

[utiberious]

Discovered during code audit in #575.

Change

Removes the and not out.is_zero() exemption added in #510, restoring the upstream Macdonald behavior where F(basis_vector) must return a proper vector.

Before (galgebra):

if not out.is_vector() and not out.is_zero():
    raise ValueError('{} must return vectors'.format(F))

After (matches upstream zip):

if not out.is_vector():
    raise ValueError('{} must return vectors'.format(F))

Rationale

A linear transform that maps any basis vector to zero is singular. Accepting it silently allows constructing an Lt that will produce incorrect results (e.g. wrong determinant, inverse undefined). Making the error explicit at construction time is safer.

[utensil]

The is_zero() exemption wasn't from #510 — it was added in #560 to fix #540. Projection maps like lambda x: (x | e1) * e1 legitimately return zero for perpendicular basis vectors, and the zero map is a valid linear map too. The existing test_lt_callable_zero regression test covers exactly this.

This change would revert a deliberate fix.

[utiberious]

Closing — the change is incorrect. The and not out.is_zero() exemption was added in #560 to fix #540: projection maps like lambda x: (x | e1) * e1 legitimately return zero for perpendicular basis vectors, and the regression test test_lt_callable_zero covers this. The upstream zip predates that fix. No change needed.