perf: use less cycles in @timer_helper (#4777)

Since the observation of a `null` in the expiration array means there will be no more found, it is pointless to stay in the loop. I expect 1 expiration per callback in the frequent case.

src/prelude/internals.mo

@@ -614,8 +614,8 @@ func @timer_helper() : async () {
 
   for (o in thunks.vals()) {
     switch o {
-      case (?thunk) { ignore thunk() };
-      case _ { }
+      case (?thunk) ignore thunk();
+      case _ return
     }
   }
 };