regress/tools-platform: add test for "$@" and ksh

regress/tools-platform/sh.test

@@ -1,5 +1,5 @@
 #! /bin/sh
-# $NetBSD: sh.test,v 1.3 2020/05/03 07:38:50 rillig Exp $
+# $NetBSD: sh.test,v 1.4 2020/05/21 13:33:35 rillig Exp $
 #
 # Tests for the shell that is available as ${SH} in Makefiles.
 #
@@ -58,3 +58,9 @@ case $MACHINE_PLATFORM in
 	assert_that "${line%%/\**}" --equals '#define bindir "/usr/bin" '
 	;;
 esac
+
+# Make sure that the shell can process empty arguments.
+#
+# For example, /bin/ksh on NetBSD 8 cannot, it complains with:
+#   ksh: @: parameter not set
+sh -eu -c ': "$@"'