/series/import/request/{id}(imageUrl): add validation for non-empty file

[0pdd]

The puzzle 709-6a277942 from #709 has to be resolved:

mystamps/src/main/java/ru/mystamps/web/feature/series/importing/ImportSeriesForm.java

Line 95 in 77cf2a7

// @todo #709 /series/import/request/{id}(imageUrl): add validation for non-empty file

The puzzle was created by Slava Semushin on 20-Jun-22

[php-coder]

Duplicate of #728