Fix for error sending e-mail with attachments

When sending an e-mail via wp_mail where the attachments array contains empty entries, the plugin will crash in mail.php line 163, 169 and 170.
This will also prevent the e-mail from being send.

This can be easily fixed by adding
$attachments = array_filter($attachments);
before the if-check on line 155 in the same file.

Hope this can be fixed in the next update.