Fallback to file completer in custom/external completer (#14781)

# Description

Closes #14595. This modifies the behavior of both custom and external
completers so that if the custom/external completer returns an invalid
value, completions are suppressed and an error is logged. However, if
the completer returns `null` (which this PR treats as a special value),
we fall back to file completions.

Previously, custom completers and external completers had different
behavior. Any time an external completer returned an invalid value
(including `null`), we would fall back to file completions. Any time a
custom completer returned an invalid value (including `null`), we would
suppress completions.

I'm not too happy about the implementation, but it's the least intrusive
way I could think of to do it. I added a `fallback` field to
`CustomCompletions` that's checked after calling its `fetch()` method.
If `fallback` is true, then we use file completions afterwards.

An alternative would be to make `CustomCompletions` no longer implement
the `Completer` trait, and instead have its `fetch()` method return an
`Option<Vec<Suggestion>>`. But that resulted in a teeny bit of code
duplication.

# User-Facing Changes

For those using an external completer, if they want to fall back to file
completions on invalid values, their completer will have to explicitly
return `null`. Returning `"foo"` or something will no longer make
Nushell use file completions instead.

For those making custom completers, they now have the option to fall
back to file completions.

# Tests + Formatting

Added some tests and manually tested that if the completer returns an
invalid value or the completer throws an error, that gets logged and
completions are suppressed.

# After Submitting

The documentation for custom completions and external completers will
have to be updated after this.

crates/nu-cli/tests/completions/mod.rs

@@ -234,6 +234,37 @@ fn customcompletions_no_sort() {
     match_suggestions(&expected, &suggestions);
 }
 
+/// Fallback to file completions if custom completer returns null
+#[test]
+fn customcompletions_fallback() {
+    let (_, _, mut engine, mut stack) = new_engine();
+    let command = r#"
+        def comp [] { null }
+        def my-command [arg: string@comp] {}"#;
+    assert!(support::merge_input(command.as_bytes(), &mut engine, &mut stack).is_ok());
+
+    let mut completer = NuCompleter::new(Arc::new(engine), Arc::new(stack));
+    let completion_str = "my-command test";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    let expected: Vec<String> = vec![folder("test_a"), file("test_a_symlink"), folder("test_b")];
+    match_suggestions(&expected, &suggestions);
+}
+
+/// Suppress completions for invalid values
+#[test]
+fn customcompletions_invalid() {
+    let (_, _, mut engine, mut stack) = new_engine();
+    let command = r#"
+        def comp [] { 123 }
+        def my-command [arg: string@comp] {}"#;
+    assert!(support::merge_input(command.as_bytes(), &mut engine, &mut stack).is_ok());
+    let mut completer = NuCompleter::new(Arc::new(engine), Arc::new(stack));
+
+    let completion_str = "my-command foo";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    assert!(suggestions.is_empty());
+}
+
 #[test]
 fn dotnu_completions() {
     // Create a new engine
@@ -312,6 +343,27 @@ fn external_completer_pass_flags() {
     assert_eq!("--", suggestions.get(2).unwrap().value);
 }
 
+/// Fallback to file completions when external completer returns null
+#[test]
+fn external_completer_fallback() {
+    let block = "{|spans| null}";
+    let input = "foo test".to_string();
+
+    let expected = vec![folder("test_a"), file("test_a_symlink"), folder("test_b")];
+    let suggestions = run_external_completion(block, &input);
+    match_suggestions(&expected, &suggestions);
+}
+
+/// Suppress completions when external completer returns invalid value
+#[test]
+fn external_completer_invalid() {
+    let block = "{|spans| 123}";
+    let input = "foo ".to_string();
+
+    let suggestions = run_external_completion(block, &input);
+    assert!(suggestions.is_empty());
+}
+
 #[test]
 fn file_completions() {
     // Create a new engine