test_mx_util: Call mx_call_external with argv != NULL

Newer Valgrind versions complain, when `execv()` is called with
argv==NULL or with argv[0]==NULL.

argv == NULL is not allowed by Posix, but the Linux kernel transforms
that into an empty argument list. An empty argument list with
argv[0]==NULL does not go against any specs, altough it might confuse
programms and led to pkexec being exploitable (CVE-2021-4034)

It is not wrong on Linux to call execv() or a wrapper function with
argv==NULL, but avoid it anyway to prevent Valgrind warnings.

test_mx_util.c

@@ -597,11 +597,13 @@ static void test_mx_call_external(void) {
     int sts;
     errno = 999;
 
-    sts = mx_call_external("/usr/bin/true", NULL);
+    char *argv[] = { "", NULL };
+
+    sts = mx_call_external("/usr/bin/true", argv);
     assert(sts == 0);
     assert(errno == 999);
 
-    sts = mx_call_external("/usr/bin/false", NULL);
+    sts = mx_call_external("/usr/bin/false", argv);
     assert(sts == -1);
     assert(errno == EPROTO);
 }