Fix null reference issue in DocumentController

Added a null-conditional operator (`?.`) to safely access the
`ByteData` property of the `senderDoc` object, preventing a
potential `NullReferenceException` when `senderDoc` is null.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Controllers/DocumentController.cs

@@ -37,7 +37,7 @@ public class DocumentController(IMediator mediator, IAuthorizationService authSe
             return BadRequest("Missing document query.");
 
         var senderDoc = await mediator.Send(query, cancel);
-        return senderDoc.ByteData is byte[] senderDocByte
+        return senderDoc?.ByteData is byte[] senderDocByte
             ? File(senderDocByte, "application/octet-stream")
             : NotFound("Document is empty.");
     }