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
         "#
     ));