Fix null reference exception in user agent handling (#3946)

Author: salmanmkc

src/Runner.Worker/JobRunner.cs

@@ -50,7 +50,7 @@ public async Task<TaskResult> RunAsync(AgentJobRequestMessage message, Cancellat
             if (message.Variables.TryGetValue(Constants.Variables.System.OrchestrationId, out VariableValue orchestrationId) &&
                 !string.IsNullOrEmpty(orchestrationId.Value))
             {
-                if (!HostContext.UserAgents.Any(x => string.Equals(x.Product.Name, "OrchestrationId", StringComparison.OrdinalIgnoreCase)))
+                if (!HostContext.UserAgents.Any(x => string.Equals(x.Product?.Name, "OrchestrationId", StringComparison.OrdinalIgnoreCase)))
                 {
                     // make the orchestration id the first item in the user-agent header to avoid get truncated in server log.
                     HostContext.UserAgents.Insert(0, new ProductInfoHeaderValue("OrchestrationId", orchestrationId.Value));