fix: 修复 DeliveryZoneService 可空警告

src/Infrastructure/TakeoutSaaS.Infrastructure/App/Services/DeliveryZoneService.cs

@@ -167,7 +167,7 @@ public sealed class DeliveryZoneService : IDeliveryZoneService
 
     private static bool TryReadPolygonFromCoordinates(JsonElement coordinatesElement, out Polygon polygon)
     {
-        polygon = default;
+        polygon = default!;
         if (!TryReadRings(coordinatesElement, out var rings) || rings.Count == 0)
         {
             return false;