refactor: replace verbose checks with String.IsNullOrEmpty where applicable.

Thanks to Kira for this patch from http://opensimulator.org/mantis/view.php?id=6845

OpenSim/Framework/Servers/HttpServer/BaseHttpServer.cs

@@ -785,7 +785,7 @@ namespace OpenSim.Framework.Servers.HttpServer
                 "[BASE HTTP SERVER]: HTTP IN {0} :{1} {2} content type handler {3} {4} from {5}",
                 RequestNumber,
                 Port,
-                (request.ContentType == null || request.ContentType == "") ? "not set" : request.ContentType,
+                string.IsNullOrEmpty(request.ContentType) ? "not set" : request.ContentType,
                 request.HttpMethod,
                 request.Url.PathAndQuery,
                 request.RemoteIPEndPoint);