Haxe Concurrent is not compatitable with haxe 5

[barisyild]

I think null safety has been improved with haxe 5, so it seems like a fix is ​​needed.

Affected target is cpp

https://github.com/hxwell/hxwell/actions/runs/17515261376/job/49751928536

[sebthom]

This is a bug in the CPP target which believes that for

      final sb = new StringBuf();
      sb.add("rethrown exception:\n");

sb could be null when calling sb.add()

Other targets work fine.

[barisyild]

cpp problem

[ERROR] D:/haxelib/haxe-concurrent/5,1,5/src/hx/concurrent/collection/SynchronizedMap.hx:260: characters 49-53

 260 |    inline public function size():Int return map.size();
     |                                                 ^^^^
     | haxe.ds.HashMap<hx.concurrent.collection._SynchronizedMap.HashMapDelegate.K, hx.concurrent.collection._SynchronizedMap.HashMapDelegate.V> has no field size