Fix exception on Internet Explorer when adding first transcript language

Internet Explorer throws an "Invalid argument" exception when uses an empty string as the parameter in ```namedItem``` method.
Change empty string into null value can maintain browser consistency.

cms/static/js/views/video/translations_editor.js

@@ -47,6 +47,13 @@ function($, _, AbstractEditor, FileUpload, UploadDialog) {
                     var dropdown = $(element).clone();
 
                     _.each(values, function(value, key) {
+                        // Note: IE may raise an exception if key is an empty string,
+                        // while other browsers return null as excepted. So coerce it
+                        // into null for browser consistency.
+                        if (key === "") {
+                            key = null;
+                        }
+
                         var option = dropdown[0].options.namedItem(key);
 
                         if (option) {