FIX: add a space after the default left prompt (#9074)

# Description
when running `nushell` with the `--no-config-file` option, the left
prompt does not have a space to separate the directory path from the
user input.
in this PR i add a space there to make the prompt easier to read when
using `--no-config-file`!

# User-Facing Changes
before: https://asciinema.org/a/581733
after: https://asciinema.org/a/581734

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- ⚫ `toolkit test`
- ⚫ `toolkit test stdlib`

# After Submitting
```
$nothing
```

crates/nu-cli/src/prompt.rs

@@ -106,11 +106,13 @@ impl Prompt for NushellPrompt {
             prompt_string.replace('\n', "\r\n").into()
         } else {
             let default = DefaultPrompt::default();
-            default
+            let prompt = default
                 .render_prompt_left()
                 .to_string()
                 .replace('\n', "\r\n")
-                .into()
+                + " ";
+
+            prompt.into()
         }
     }