Return external file completions if not empty (#10898)

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# Description
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If an external completer is used and it returns no completions for a
filepath, we fall back to the builtin path completer.

# User-Facing Changes
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helps us keep track of breaking changes. -->

Path completions will remain consistent with the use of an external
completer.

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

@@ -347,7 +347,9 @@ impl NuCompleter {
                                             if let Some(external_result) = self.external_completion(
                                                 block_id, &spans, offset, new_span,
                                             ) {
-                                                return external_result;
+                                                if !external_result.is_empty() {
+                                                    return external_result;
+                                                }
                                             }
                                         }