Replace use of $nothing with null (#621)

`$nothing` will be deprecated in nu 0.86

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

custom-completions/auto-generate/parse-fish.nu

@@ -27,7 +27,7 @@ def parse-fish [] {
         | flatten                                 # merge them all into a top level label
     )
     # default every column in the table to "" to make processing easier
-    # some values having $nothing often breaks nu or requires lots of checking
+    # some values having null often breaks nu or requires lots of checking
     $data | columns | reduce -f $data { |c, acc|
         $acc | default "" $c
     }

custom-completions/winget/winget-completions.nu

@@ -374,7 +374,7 @@ def "nu-complete winget source type" [] {
 
 def "nu-complete winget flagify" [name: string, value: any, --short(-s): bool] {
   let flag_start = if $short { '-' } else { '--' }
-  if $value == $nothing or $value == false {
+  if $value == null or $value == false {
     ""
   } else if $value == true {
     $"($flag_start)($name)"