Replace use of $nothing with null (#621)

`$nothing` will be deprecated in nu 0.86

This accompanies
- https://github.com/nushell/nushell/pull/10478

benchmarks/gradient-autoview.nu

@@ -16,5 +16,5 @@ seq 0 $height | par-each {|| # create these in parallel
         }
     } | str join)
     print -n $"($row_data)" | table
-    $nothing
+    null
 } | compact