Be more defensive in BufferingDisabled test (#12278)

Fixes https://github.com/aspnet/AspNetCore-Internal/issues/2826

We [use `?.` in `master`](f56cb72b7f/src/Hosting/Server.IntegrationTesting/src/Deployers/ApplicationDeployer.cs (L103)) and it seems like the test can end up in this state (by the linked issue) so back-porting that defensive measure seems reasonable.

src/Hosting/Server.IntegrationTesting/src/Deployers/ApplicationDeployer.cs

@@ -97,7 +97,7 @@ namespace Microsoft.AspNetCore.Server.IntegrationTesting
                 }
                 else
                 {
-                    _publishedApplication.Dispose();
+                    _publishedApplication?.Dispose();
                 }
             }
         }