add plugin path when there are no signatures (#10201)

# Description

This PR adds the ability to have a `$nu.plugin-path` even when you
plugins are registered and it also should work with `nu -n --no-stdlib`.

### Before
It would give an error
```
│ plugin-path        │ IOError("Could not get plugin signature location")             │
```

### After
It returns the proper path, like this for me
```
│ plugin-path        │ /Users/fdncred/Library/Application Support/nushell/plugin.nu   │
```

Closed #10198 
# User-Facing Changes
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crates/nu-protocol/src/eval_const.rs

@@ -113,6 +113,11 @@ pub fn create_nu_constant(engine_state: &EngineState, span: Span) -> Result<Valu
             if let Some(path) = &engine_state.plugin_signatures {
                 let canon_plugin_path = canonicalize_path(engine_state, path);
                 Value::string(canon_plugin_path.to_string_lossy(), span)
+            } else if let Some(mut plugin_path) = nu_path::config_dir() {
+                // If there are no signatures, we should still populate the plugin path
+                plugin_path.push("nushell");
+                plugin_path.push("plugin.nu");
+                Value::string(plugin_path.to_string_lossy(), span)
             } else {
                 Value::error(
                     ShellError::IOError("Could not get plugin signature location".into()),