[FIX] web_editor: properly save media dialog without any image

When saving the media dialog without any selected image, a crash
occurred. Now, it properly closes the dialog.

opw-1858614

addons/web_editor/static/src/js/widgets.js

@@ -326,8 +326,7 @@ var ImageDialog = Widget.extend({
 
         var img = this.images[0];
         if (!img) {
-            var id = this.$(".existing-attachments [data-src]:first").data('id');
-            img = _.find(this.images, function (img) { return img.id === id;});
+            return this.media;
         }
 
         if (!img.is_document) {