Disable "Override" if we have no fields to override

We already have this functionality, but there is error.

Look at this code:

if (conf && conf.modes && !(conf.modes.length === 1 && conf.modes.full && !conf.fields.length)) {
  menu.assetoverride = CKEDITOR.TRISTATE_ON;
}

We should disable the override functionality if we have one value or have not values in conf.modes. But when we have one view mode in conf.modes, it looks like {full: "Full size"}. We can't call .length from object, it returns undefined.