Support pattern matching null literals (nushell#10829)

# Description
Support pattern matching against the `null` literal.  Fixes nushell#10799 

### Before
```nushell
> match null { null => "success", _ => "failure" }
failure
```

### After
```nushell
> match null { null => "success", _ => "failure" }
success
```

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
Users can pattern match against a `null` literal.

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

@@ -132,6 +132,12 @@ fn match_constant_7() {
     assert_eq!(actual.out, "success");
 }
 
+#[test]
+fn match_null() {
+    let actual = nu!(r#"match null { null => { print "success"}, _ => { print "failure" }}"#);
+    assert_eq!(actual.out, "success");
+}
+
 #[test]
 fn match_or_pattern() {
     let actual = nu!(

crates/nu-protocol/src/engine/pattern_match.rs

@@ -96,6 +96,9 @@ impl Matcher for Pattern {
             Pattern::Value(pattern_value) => {
                 // TODO: Fill this out with the rest of them
                 match &pattern_value.expr {
+                    Expr::Nothing => {
+                        matches!(value, Value::Nothing { .. })
+                    }
                     Expr::Int(x) => {
                         if let Value::Int { val, .. } = &value {
                             x == val