Add a new error message when the valid_fields is empty

> "Attribute `{}` is not sortable. This index doesn't have configured sortable attributes."
> "Attribute `{}` is not sortable. Available sortable attributes are: `{}`."

coexist in the error handling

milli/src/error.rs

@@ -256,12 +256,21 @@ only composed of alphanumeric characters (a-z A-Z 0-9), hyphens (-) and undersco
             Self::InvalidSortableAttribute { field, valid_fields } => {
                 let valid_names =
                     valid_fields.iter().map(AsRef::as_ref).collect::<Vec<_>>().join(", ");
+
+                    if valid_names.is_empty() {
+                        write!(
+                            f,
+                            "Attribute `{}` is not sortable. This index does not have configured sortable attributes.",
+                            field
+                        )
+                    } else {
                         write!(
                             f,
                             "Attribute `{}` is not sortable. Available sortable attributes are: `{}`.",
                             field, valid_names
                         )
                     }
+            }
             Self::SortRankingRuleMissing => f.write_str(
                 "The sort ranking rule must be specified in the \
 ranking rules settings to use the sort parameter at search time.",
@@ -320,3 +329,19 @@ impl fmt::Display for SerializationError {
 }
 
 impl StdError for SerializationError {}
+
+#[test]
+fn conditionally_lookup_for_error_message() {
+    let prefix = "Attribute `name` is not sortable.";
+    let messages = vec![
+        (BTreeSet::new(), "This index does not have configured sortable attributes."),
+        (BTreeSet::from(["age".to_string()]), "Available sortable attributes are: `age`."),
+    ];
+
+    for (list, suffix) in messages {
+        let err =
+            UserError::InvalidSortableAttribute { field: "name".to_string(), valid_fields: list };
+
+        assert_eq!(err.to_string(), format!("{} {}", prefix, suffix));
+    }
+}