fix npe for WaitForInstanceCompletion

Author: kaibocai

src/WebJobs.Extensions.DurableTask/LocalGrpcListener.cs

@@ -287,6 +287,11 @@ await this.GetDurabilityProvider(context).ForceTerminateTaskOrchestrationAsync(
                     timeout: Timeout.InfiniteTimeSpan,
                     context.CancellationToken);
 
+                if (state == null)
+                {
+                    return new P.GetInstanceResponse() { Exists = false };
+                }
+
                 return CreateGetInstanceResponse(state, request);
             }