fix(driver-historian-wonderware): resolve Medium code-review finding (Driver.Historian.Wonderware-002)

Normalise req.Events to Array.Empty<AlarmHistorianEventDto>() immediately
after MessagePack deserialization in HandleWriteAlarmEventsAsync. MessagePack
deserializes an absent or explicit-nil array field as null, not Array.Empty,
so a peer that sends a null Events array would trigger a NullReferenceException
on either .Length dereference (no-writer branch or catch block), leaving the
client correlation-id wait hanging with no reply frame.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware/Ipc/HistorianFrameHandler.cs

@@ -153,6 +153,11 @@ public sealed class HistorianFrameHandler : IFrameHandler
     private async Task HandleWriteAlarmEventsAsync(byte[] body, FrameWriter writer, CancellationToken ct)
     {
         var req = MessagePackSerializer.Deserialize<WriteAlarmEventsRequest>(body);
+
+        // MessagePack deserializes an absent or explicit-nil array as null, not Array.Empty.
+        // Normalise here so every path below can safely dereference .Length without an NRE.
+        req.Events ??= Array.Empty<AlarmHistorianEventDto>();
+
         var reply = new WriteAlarmEventsReply { CorrelationId = req.CorrelationId };
 
         if (_alarmWriter is null)