fix(node): pop and flatten the result instead of indexing

crates/iota-json-rpc/src/read_api.rs

@@ -793,7 +793,9 @@ impl ReadApiServer for ReadApi {
                 .map_err(|e| {
                     error!("Failed to retrieve checkpoint sequence for transaction {digest:?} with error: {e:?}");
                     Error::from(e)
-                })?[0];
+                })?
+                .pop()
+                .flatten();
 
             if let Some(checkpoint_seq) = &temp_response.checkpoint_seq {
                 let kv_store = self.transaction_kv_store.clone();