[.NET] Temporary handling of "not found" schema registry error (#602)

Author: timtay-microsoft

dotnet/src/Azure.Iot.Operations.Services/SchemaRegistry/SchemaRegistryClient.cs

@@ -36,6 +36,12 @@ public class SchemaRegistryClient(ApplicationContext applicationContext, IMqttPu
                     }
                 }, null, null, timeout ?? s_DefaultCommandTimeout, cancellationToken)).Schema;
         }
+        catch (AkriMqttException ex) when (ex.Kind == AkriMqttErrorKind.PayloadInvalid)
+        {
+            // This is likely because the user received a "not found" response payload from the service, but the service is an
+            // older version that sends an empty payload instead of the expected "{}" payload.
+            return null;
+        }
         catch (AkriMqttException e) when (e.Kind == AkriMqttErrorKind.UnknownError)
         {
             // ADR 15 specifies that schema registry clients should still throw a distinct error when the service returns a 422. It also specifies