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/SIMPLE2

@@ -97,7 +97,7 @@
 
             dcl-s len uns(10);
 
-            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;