don't allow exec or spawn with an argument vector of just NULL.
we already rejected argv == NULL itself, but this was allowed:
   char *argv[1] = { 0 };

Index: kern_exec.c
===================================================================
RCS file: /cvsroot/src/sys/kern/kern_exec.c,v
retrieving revision 1.514
diff -p -u -r1.514 kern_exec.c
--- kern_exec.c	26 Nov 2021 08:06:12 -0000	1.514
+++ kern_exec.c	30 Jan 2022 03:24:42 -0000
@@ -1666,6 +1666,10 @@ copyinargs(struct execve_data * restrict
 		return error;
 	}
 	data->ed_argc += i;
+	if (data->ed_argc == 0) {
+		DPRINTF(("%s: zero args\n", __func__));
+		return EINVAL;
+	}
 
 	/*
 	 * Read and count environment strings from user.