fix null analysis issues on Haxe 4.3

Author: sebthom

src/hx/concurrent/collection/SynchronizedLinkedList.hx

@@ -325,6 +325,7 @@ private class SynchronizedLinkedListImpl<T> implements OrderedCollection<T> {
          var i = 0;
          var foundAt = -1;
          for (item in _items) {
+            #if (haxe_ver >= 4.3) @:nullSafety(Off) #end
             if (startAt != null && i > startAt)
                break;
             if (item == x)

src/hx/concurrent/executor/ThreadPoolExecutor.hx

@@ -120,14 +120,12 @@ class ThreadPoolExecutor extends Executor {
    }
 
 
-   public function submit<T>(task:Either2<Void->T,Void->Void>, ?schedule:Schedule):TaskFuture<T>
+   public function submit<T>(task:Either2<Void->T,Void->Void>, ?schedule:Schedule):TaskFuture<T> {
+      final schedule:Schedule = schedule == null ? Executor.NOW_ONCE : schedule;
       return _stateLock.execute(function() {
          if (state != RUNNING)
             throw 'Cannot accept new tasks. Executor is not in state [RUNNING] but [$state].';
 
-         if (schedule == null)
-            schedule = Executor.NOW_ONCE;
-
          final future = new TaskFutureImpl<T>(this, task, schedule);
 
          // skip round-trip via scheduler for one-shot tasks that should be executed immediately
@@ -143,6 +141,7 @@ class ThreadPoolExecutor extends Executor {
          _newScheduledTasks.push(future);
          return future;
       });
+   }
 
 
    override //

src/hx/concurrent/executor/TimerExecutor.hx

@@ -5,7 +5,6 @@
  */
 package hx.concurrent.executor;
 
-import hx.concurrent.Future.FutureResult;
 import hx.concurrent.executor.Executor.AbstractTaskFuture;
 import hx.concurrent.executor.Executor.Task;
 import hx.concurrent.executor.Executor.TaskFuture;
@@ -30,7 +29,8 @@ class TimerExecutor extends Executor {
    }
 
 
-   public function submit<T>(task:Either2<Void->T, Void->Void>, ?schedule:Schedule):TaskFuture<T>
+   public function submit<T>(task:Either2<Void->T, Void->Void>, ?schedule:Schedule):TaskFuture<T> {
+      final schedule:Schedule = schedule == null ? Executor.NOW_ONCE : schedule;
       return _stateLock.execute(function() {
          if (state != RUNNING)
             throw 'Cannot accept new tasks. Executor is not in state [RUNNING] but [$state].';
@@ -40,14 +40,14 @@ class TimerExecutor extends Executor {
          while (i-- > 0)
             if (_scheduledTasks[i].isStopped) _scheduledTasks.splice(i, 1);
 
-         final future = new TaskFutureImpl<T>(this, task, schedule == null ? Executor.NOW_ONCE : schedule);
+         final future = new TaskFutureImpl<T>(this, task, schedule);
          switch (schedule) {
             case ONCE(0):
             default: _scheduledTasks.push(future);
          }
          return future;
       });
-
+   }
 
    override //
    function onStop() {