update search css loading

MediaBrowser.Api/Library/LibraryHelpers.cs

@@ -56,6 +56,11 @@ namespace MediaBrowser.Api.Library
         /// <param name="appPaths">The app paths.</param>
         public static void AddMediaPath(IFileSystem fileSystem, string virtualFolderName, string path, IServerApplicationPaths appPaths)
         {
+            if (!string.IsNullOrWhiteSpace(path))
+            {
+                throw new ArgumentNullException("path");
+            }
+
 			if (!fileSystem.DirectoryExists(path))
             {
                 throw new DirectoryNotFoundException("The path does not exist.");