Remove default list-diving behaviour (#13386)

# Description

Before this change `default` would dive into lists and replace `null`
elements with the default value.
Therefore there was no easy way to process `null|list<null|string>`
input and receive `list<null|string>`
(IOW to apply the default on the top level only).

However it's trivially easy to apply default values to list elements
with the `each` command:
```nushell
[null, "a", null] | each { default "b" }
```

So there's no need to have this behavior in `default` command.

# User-Facing Changes

* `default` no longer dives into lists to replace `null` elements with
the default value.

# Tests + Formatting

Added a couple of tests for the new (and old) behavior.

# After Submitting

* Update docs.

crates/nu-command/src/filters/default.rs

@@ -51,11 +51,11 @@ impl Command for Default {
                 description:
                     "Get the env value of `MY_ENV` with a default value 'abc' if not present",
                 example: "$env | get --ignore-errors MY_ENV | default 'abc'",
-                result: None, // Some(Value::test_string("abc")),
+                result: Some(Value::test_string("abc")),
             },
             Example {
                 description: "Replace the `null` value in a list",
-                example: "[1, 2, null, 4] | default 3",
+                example: "[1, 2, null, 4] | each { default 3 }",
                 result: Some(Value::list(
                     vec![
                         Value::test_int(1),
@@ -113,15 +113,7 @@ fn default(
     } else if input.is_nothing() {
         Ok(value.into_pipeline_data())
     } else {
-        input
-            .map(
-                move |item| match item {
-                    Value::Nothing { .. } => value.clone(),
-                    x => x,
-                },
-                engine_state.signals(),
-            )
-            .map(|x| x.set_metadata(metadata))
+        Ok(input)
     }
 }
 

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

@@ -32,3 +32,15 @@ fn default_after_empty_filter() {
 
     assert_eq!(actual.out, "d");
 }
+
+#[test]
+fn keeps_nulls_in_lists() {
+    let actual = nu!(r#"[null, 2, 3] | default [] | to json -r"#);
+    assert_eq!(actual.out, "[null,2,3]");
+}
+
+#[test]
+fn replaces_null() {
+    let actual = nu!(r#"null | default 1"#);
+    assert_eq!(actual.out, "1");
+}