[test-stress-options] ensure max is not called with empty list

some reports e.g. sample-graphs.scm may have no options; this commit
causes max to be called with at least 1 parameter (max 0) --> 0.

gnucash/report/reports/standard/test/test-stress-options.scm

@@ -141,8 +141,9 @@
              #f))))
      (iota
       (apply max
-             (map (lambda (opt) (length (vector-ref opt 3)))
-                  report-options)))
+             (cons 0
+                   (map (lambda (opt) (length (vector-ref opt 3)))
+                        report-options))))
      )))
 
 (define (combinatorial-stress-test report-name uuid report-options)