Revert "Return external file completions if not empty (#10898)" (#11446)

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This reverts #10898 which breaks external completion.
Not having file completion fallback on empty result is **intentional**
as this indicates that there is nothing to complete at this position.
To have nushell fallback to file completion the external completer can
simply return *nothing*.


`NO RECORDS FOUND`:
```nushell
let external_completer = {|spans|
    []
}
```

Fallback to file completion:
```nushell
let external_completer = {|spans|
}
```

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

@@ -354,9 +354,7 @@ impl NuCompleter {
                                             if let Some(external_result) = self.external_completion(
                                                 block_id, &spans, offset, new_span,
                                             ) {
-                                                if !external_result.is_empty() {
-                                                    return external_result;
-                                                }
+                                                return external_result;
                                             }
                                         }