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-command/tests/commands/str_/into_string.rs

@@ -165,7 +165,7 @@ fn from_nothing() {
     let actual = nu!(
         cwd: ".", pipeline(
         r#"
-        $nothing | into string
+        null | into string
         "#
     ));
 

crates/nu-command/tests/commands/with_env.rs

@@ -72,7 +72,7 @@ fn with_env_hides_variables_in_parent_scope() {
         r#"
         let-env FOO = "1"
         echo $env.FOO
-        with-env [FOO $nothing] {
+        with-env [FOO null] {
             echo $env.FOO
         }
         echo $env.FOO
@@ -91,10 +91,10 @@ fn with_env_shorthand_can_not_hide_variables() {
         r#"
         let-env FOO = "1"
         echo $env.FOO
-        FOO=$nothing echo $env.FOO
+        FOO=null echo $env.FOO
         echo $env.FOO
         "#
     );
 
-    assert_eq!(actual.out, "1$nothing1");
+    assert_eq!(actual.out, "1null1");
 }

crates/nu-parser/tests/test_parser.rs

@@ -323,10 +323,10 @@ pub fn parse_call_missing_req_flag() {
 }
 
 #[test]
-fn test_nothing_comparisson_eq() {
+fn test_nothing_comparison_eq() {
     let engine_state = EngineState::new();
     let mut working_set = StateWorkingSet::new(&engine_state);
-    let (block, err) = parse(&mut working_set, None, b"2 == $nothing", true, &[]);
+    let (block, err) = parse(&mut working_set, None, b"2 == null", true, &[]);
 
     assert!(err.is_none());
     assert!(block.len() == 1);
@@ -346,10 +346,10 @@ fn test_nothing_comparisson_eq() {
 }
 
 #[test]
-fn test_nothing_comparisson_neq() {
+fn test_nothing_comparison_neq() {
     let engine_state = EngineState::new();
     let mut working_set = StateWorkingSet::new(&engine_state);
-    let (block, err) = parse(&mut working_set, None, b"2 != $nothing", true, &[]);
+    let (block, err) = parse(&mut working_set, None, b"2 != null", true, &[]);
 
     assert!(err.is_none());
     assert!(block.len() == 1);

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: {