fix(completion): invalid prefix for external path argument with spaces (#15998)

Fixes the default behavior of #15790 

# Description

As for the mentioned carapace version: `cat ~"/Downloads/Obsidian
Vault/"`, the problem lies in the unexpanded home directory `~`. Either
we encourage users to manually expand that in
`$env.config.completions.external.completer` or open an issue on the
carapace project.

# User-Facing Changes

bug fix

# Tests + Formatting

Adjusted

# After Submitting

crates/nu-cli/src/completions/completer.rs

@@ -466,6 +466,14 @@ impl NuCompleter {
                                 return suggestions;
                             }
                         }
+                        // for external path arguments with spaces, please check issue #15790
+                        if suggestions.is_empty() {
+                            let (new_span, prefix) =
+                                strip_placeholder_if_any(working_set, &span, strip);
+                            let ctx = Context::new(working_set, new_span, prefix, offset);
+                            suggestions.extend(self.process_completion(&mut FileCompletion, &ctx));
+                            return suggestions;
+                        }
                         break;
                     }
                 }