`matrix-cos-angle` divides by zero on zero matrices

[samth]

matrix-cos-angle raises a /: division by zero error when either argument is a zero matrix:

> (matrix-cos-angle (matrix [[0]]) (matrix [[1]]))
/: division by zero

Found by DrDr here: https://drdr.racket-lang.org/72356/racket/share/pkgs/math-test/math/tests/matrix-tests.rkt

This code in matrix-basic.rkt looks suspicious:

[(or (nan? mxA) (nan? mxB) (= 0 mxA) (= 0 mxB))
 (/ (matrix-dot A B) (* mxA mxB))]

When mxA or mxB is 0, (* mxA mxB) is 0, causing division by zero.

I don't know what it should return, though.

[bdeket]

this seems ± correct to me.
otherwise another error should be returned (you can not measure an angle between a vector and a zero vector / matrix and zero matrix.)

[samth]

I think at a minimum the test and error message should change. I don't have a strong feeling otherwise.