Add AppOfflineDroppedWhileSiteIsDown_SiteReturns503_InProcess to test retry list (#41718)

Author: BrennanConroy

eng/test-configuration.json

@@ -4,6 +4,7 @@
   "localRerunCount" : 3,
   "retryOnRules": [
     {"testName": {"contains": "AppOfflineDroppedWhileSiteStarting_SiteShutsDown_InProcess"}},
+    {"testName": {"contains": "AppOfflineDroppedWhileSiteIsDown_SiteReturns503_InProcess"}},
     {"testName": {"contains": "CheckFrebDisconnect"}},
     {"testName": {"contains": "CheckStdoutWithLargeWrites"}},
     {"testName": {"contains": "ServerShutsDownWhenMainExitsStress" }},

src/Servers/IIS/IntegrationTesting.IIS/src/IISDeployer.cs

@@ -264,7 +264,7 @@ private void WaitUntilSiteStarted(string contentRoot)
             var workerProcess = appPool.WorkerProcesses.SingleOrDefault();
             if (workerProcess == null)
             {
-                throw new InvalidOperationException("Site is started but no worked process found");
+                throw new InvalidOperationException("Site is started but no worker process found");
             }
 
             HostProcess = Process.GetProcessById(workerProcess.ProcessId);
@@ -413,7 +413,7 @@ private void Stop()
                     }
                 }
 
-                if (!HostProcess.HasExited)
+                if (HostProcess is not null && !HostProcess.HasExited)
                 {
                     throw new InvalidOperationException("Site is stopped but host process is not");
                 }