add null check for protocol AppMetadata assignment

Author: cmettler

csharp/src/Apache.Arrow.Flight/FlightInfo.cs

@@ -76,10 +76,14 @@ internal Protocol.FlightInfo ToProtocol()
                 FlightDescriptor = Descriptor.ToProtocol(),
                 TotalBytes = TotalBytes,
                 TotalRecords = TotalRecords,
-                Ordered = Ordered,
-                AppMetadata = AppMetadata
+                Ordered = Ordered
             };
 
+            if (AppMetadata != null)
+            {
+                response.AppMetadata = AppMetadata;
+            }
+
             foreach(var endpoint in Endpoints)
             {
                 response.Endpoint.Add(endpoint.ToProtocol());