Fix sending e-mail if no errors happened for ping targets

[AggregateErrorMailService#L43](https://github.com/datasharingframework/dsf-process-ping-pong/blob/main/src/main/java/dev/dsf/bpe/mail/AggregateErrorMailService.java#L43) should have an additonal check: `sendProcessFailedMail && !errorsPerTarget.isEmpty()`