Switch to !empty instead of isset

[garex]

Helpful in case of corrupted attachment properties.

Minor hotfix.

What we recently found is that when autoresend from gmail to yandex -- someone "kills" filename property of attachments. It becomes just empty. So instead of isset it's better use !empty.

Code coverage not changed. Tests was run locally.

ps: Btw, it's better to switch to docker already ))

[garex]

TRavis failed as of it fails in master/develop too.