Handle missing PsPdfKitAnnotation with blank JSON

Refactored DocStatusHandler to assign a blank JSON object ("{}")
to the Value property when PsPdfKitAnnotation is not present or
not of the expected type, preventing potential runtime errors.

EnvelopeGenerator.Application/Common/Notifications/DocSigned/Handlers/DocStatusHandler.cs

@@ -10,6 +10,8 @@ namespace EnvelopeGenerator.Application.Common.Notifications.DocSigned.Handlers;
 /// </summary>
 public class DocStatusHandler : INotificationHandler<DocSignedNotification>
 {
+    private const string BlankAnnotationJson = "{}";
+
     private readonly ISender _sender;
 
     /// <summary>
@@ -33,7 +35,9 @@ public class DocStatusHandler : INotificationHandler<DocSignedNotification>
         {
             Envelope = new() { Id = notification.EnvelopeId },
             Receiver = new() { Id = notification.ReceiverId},
-            Value = JsonSerializer.Serialize(notification.PsPdfKitAnnotation.Instant, Format.Json.ForAnnotations)
+            Value = notification.PsPdfKitAnnotation is PsPdfKitAnnotation annot 
+                    ? JsonSerializer.Serialize(annot.Instant, Format.Json.ForAnnotations) 
+                    : BlankAnnotationJson
         }, cancel);
     }
 }