Fix slice indexing (#6322)

* Return empty suggestions if no span contents is present

* Fix slice indexing

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

@@ -81,6 +81,9 @@ impl CommandCompletion {
         match_algorithm: MatchAlgorithm,
     ) -> Vec<Suggestion> {
         let partial = working_set.get_span_contents(span);
+        if partial.is_empty() {
+            return Vec::new();
+        }
 
         let filter_predicate = |command: &[u8]| match_algorithm.matches_u8(command, partial);