Change other instances of $nothing to null (#7569)

# Description

Purely for consistency, various remaining instances of `$nothing`
(almost all of which were in test code) have been changed to `null`.
Now, the only place that refers to `$nothing` is the parser code which
implements it.

# User-Facing Changes

The default config.nu now uses `null` in certain places where it used
`$nothing`.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-utils/src/sample_config/default_config.nu

@@ -408,14 +408,14 @@ let-env config = {
 
   hooks: {
     pre_prompt: [{
-      $nothing  # replace with source code to run before the prompt is shown
+      null  # replace with source code to run before the prompt is shown
     }]
     pre_execution: [{
-      $nothing  # replace with source code to run before the repl input is run
+      null  # replace with source code to run before the repl input is run
     }]
     env_change: {
       PWD: [{|before, after|
-        $nothing  # replace with source code to run if the PWD environment is different since the last repl input
+        null  # replace with source code to run if the PWD environment is different since the last repl input
       }]
     }
     display_output: {