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.

src/tests/test_custom_commands.rs

@@ -114,7 +114,7 @@ fn def_with_no_dollar() -> TestResult {
 #[test]
 fn allow_missing_optional_params() -> TestResult {
     run_test(
-        "def foo [x?:int] { if $x != $nothing { $x + 10 } else { 5 } }; foo",
+        "def foo [x?:int] { if $x != null { $x + 10 } else { 5 } }; foo",
         "5",
     )
 }

src/tests/test_engine.rs

@@ -75,7 +75,7 @@ fn earlier_errors() -> TestResult {
 #[test]
 fn missing_flags_are_nothing() -> TestResult {
     run_test(
-        r#"def foo [--aaa(-a): int, --bbb(-b): int] { (if $aaa == $nothing { 10 } else { $aaa }) + (if $bbb == $nothing { 100 } else { $bbb }) }; foo"#,
+        r#"def foo [--aaa(-a): int, --bbb(-b): int] { (if $aaa == null { 10 } else { $aaa }) + (if $bbb == null { 100 } else { $bbb }) }; foo"#,
         "110",
     )
 }
@@ -83,7 +83,7 @@ fn missing_flags_are_nothing() -> TestResult {
 #[test]
 fn missing_flags_are_nothing2() -> TestResult {
     run_test(
-        r#"def foo [--aaa(-a): int, --bbb(-b): int] { (if $aaa == $nothing { 10 } else { $aaa }) + (if $bbb == $nothing { 100 } else { $bbb }) }; foo -a 90"#,
+        r#"def foo [--aaa(-a): int, --bbb(-b): int] { (if $aaa == null { 10 } else { $aaa }) + (if $bbb == null { 100 } else { $bbb }) }; foo -a 90"#,
         "190",
     )
 }
@@ -91,7 +91,7 @@ fn missing_flags_are_nothing2() -> TestResult {
 #[test]
 fn missing_flags_are_nothing3() -> TestResult {
     run_test(
-        r#"def foo [--aaa(-a): int, --bbb(-b): int] { (if $aaa == $nothing { 10 } else { $aaa }) + (if $bbb == $nothing { 100 } else { $bbb }) }; foo -b 45"#,
+        r#"def foo [--aaa(-a): int, --bbb(-b): int] { (if $aaa == null { 10 } else { $aaa }) + (if $bbb == null { 100 } else { $bbb }) }; foo -b 45"#,
         "55",
     )
 }
@@ -99,7 +99,7 @@ fn missing_flags_are_nothing3() -> TestResult {
 #[test]
 fn missing_flags_are_nothing4() -> TestResult {
     run_test(
-        r#"def foo [--aaa(-a): int, --bbb(-b): int] { (if $aaa == $nothing { 10 } else { $aaa }) + (if $bbb == $nothing { 100 } else { $bbb }) }; foo -a 3 -b 10000"#,
+        r#"def foo [--aaa(-a): int, --bbb(-b): int] { (if $aaa == null { 10 } else { $aaa }) + (if $bbb == null { 100 } else { $bbb }) }; foo -a 3 -b 10000"#,
         "10003",
     )
 }

src/tests/test_table_operations.rs

@@ -173,9 +173,9 @@ fn update_cell_path_1() -> TestResult {
 
 #[test]
 fn missing_column_fills_in_nothing() -> TestResult {
-    // The empty value will be replaced with $nothing when fetching a column
+    // The empty value will be replaced with null when fetching a column
     run_test(
-        r#"[ { name: ABC, size: 20 }, { name: HIJ } ].size.1 == $nothing"#,
+        r#"[ { name: ABC, size: 20 }, { name: HIJ } ].size.1 == null"#,
         "true",
     )
 }
@@ -258,7 +258,7 @@ fn length_defaulted_columns() -> TestResult {
 
 #[test]
 fn get_fuzzy() -> TestResult {
-    run_test("(ls | get -i foo) == $nothing", "true")
+    run_test("(ls | get -i foo) == null", "true")
 }
 
 #[test]