Hide "schedule" text if version is not present.

Sometimes the conference content team does not fill out the schedule version field. As far as I could see it is not used for updating the schedule on the client side. The client behavior relies on an HTTP status `HTTP_NOT_MODIFIED` aka. `304` response sent out by the server.

* [HTTP response processing](https://github.com/tuxmobil/CampFahrplan/blob/22234364b3af61debb8253ea0f1f237cf7c6d66f/app/src/main/java/nerd/tuxmobil/fahrplan/congress/FetchFahrplan.java#L175)
* [Up-to-date message](https://github.com/tuxmobil/CampFahrplan/blob/22234364b3af61debb8253ea0f1f237cf7c6d66f/app/src/main/java/nerd/tuxmobil/fahrplan/congress/CustomHttpClient.java#L192)

In questions is what triggers this HTTP response in [Frab](https://github.com/manno/frab)? Maybe, @manno can answer this question? Should this field be an mandatory field?