Exclude checking the contract for block-diagonal-matrix/zero (#107)

* Fix the contract of `block-diagonal-matrix/zero`

* Change the contract of `block-diagonal-matrix/zero` back

* Exclude checking the contract for `block-diagonal-matrix/zero`

Author: NoahStoryM

math-test/math/tests/matrix-untyped-tests.rkt

@@ -9,7 +9,8 @@
 
 (begin-for-syntax
   (define exceptions (list 'matrix 'col-matrix 'row-matrix
-                           'matrix-determinant/row-reduction))
+                           'matrix-determinant/row-reduction
+                           'block-diagonal-matrix/zero))
 
   (define (looks-like-value? sym)
     (define str (symbol->string sym))