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-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);