AddSimilarSeriesForm: seriesId must exist

The puzzle `1280-50f25bf4` from #1280 has to be resolved: 

https://github.com/php-coder/mystamps/blob/139115290d785550f58866d6add413968b49efb9/src/main/java/ru/mystamps/web/feature/series/AddSimilarSeriesForm.java#L30-L30

The puzzle was created by Slava Semushin on 12-Apr-20.
