spelling: call it null-terminate consistently

With a dash, using two Ls. Also for different forms of the word.

Use NULL in all uppercase if it means a zero pointer.

Follow-up to 307b7543ea

Closes #17489

packages/OS400/rpg-examples/HTTPPOST

@@ -118,7 +118,7 @@
       *
      d len             s             10u 0
       *
-        len = %scan(X'00': string: 1: length); // Limit to zero-terminated string
+        len = %scan(X'00': string: 1: length); // Limit to null-terminated string
         if len = 0;
             len = length + 1;
         endif;