add $nothing and tests (#2995)

crates/nu-engine/src/evaluate/evaluator.rs

@@ -253,6 +253,11 @@ fn evaluate_reference(name: &str, ctx: &EvaluationContext, tag: Tag) -> Result<V
             )),
         },
 
+        "$nothing" => Ok(Value {
+            value: UntaggedValue::nothing(),
+            tag,
+        }),
+
         x => match ctx.scope.get_var(x) {
             Some(v) => Ok(v),
             None => Err(ShellError::labeled_error(

crates/nu-engine/tests/evaluate/invocation.rs

@@ -11,3 +11,26 @@ fn test_parse_invocation_with_range() {
     );
     assert_eq!(actual.out, "[1,2,3]")
 }
+
+#[test]
+fn create_nothing_in_table() {
+    let actual = nu!(
+        cwd: ".",
+        r#"
+        echo [[column]; [$nothing]] | to json
+        "#
+    );
+    assert_eq!(actual.out, "{\"column\":null}");
+}
+
+#[test]
+fn compare_to_nothing() {
+    let actual = nu!(
+        cwd: ".",
+        r#"
+        let f = $nothing
+        if $f == $nothing {echo $true} {echo $false}
+        "#
+    );
+    assert_eq!(actual.out, "true");
+}