Fix command missing hook default config (#8540)

# Description

Unbreak unit tests by updating default config for the new hook that
landed after the syntax change but didn't get its tests re-run.

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# 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

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# 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-utils/src/sample_config/default_config.nu

@@ -304,7 +304,7 @@ let-env config = {
     display_output: {||
       if (term size).columns >= 100 { table -e } else { table }
     }
-    command_not_found: {
+    command_not_found: {||
       null  # replace with source code to return an error message when a command is not found
     }
   }