[Narwhal] Return immediately when all missing digests have been found…

… in synchronizer (MystenLabs#9331)

## Description 

Should save 1s per request where batch does not exist locally because of
delay before retry if we just break from loop.

## Test Plan 

Unit tests

narwhal/worker/src/handlers.rs

@@ -238,7 +238,7 @@ impl<V: TransactionValidator> PrimaryToWorker for PrimaryReceiverHandler<V> {
                             }
                         }
                         if missing.is_empty() {
-                            break;
+                            return Ok(anemo::Response::new(()));
                         }
                     }
                     Err(e) => {