parseInactiveRecipients does not handle empty results correctly

[TKDBB84]

Occasionally the server returns a message with no emails included. This is a separate issue, however the parseInactiveRecipients function incorrectly returns a '.' instead of an empty array in this case. When an email that isn't valid is attempted to be sent the postmark server sometimes returns:

{
  ...
  code: 406,
  message: "You tried to send to recipient(s) that have been marked as inactive. Found inactive addresses: . Inactive recipients are ones that have generated a hard bounce, a spam complaint, or a manual suppression."
  ...
}

passing the message through parseInactiveRecipients results incorrectly in: [ '.' ]. When the expected result should be [].

Minimal Replication:

> const { Errors: postmarkErrors } = require('postmark')
> postmarkErrors.InactiveRecipientsError.parseInactiveRecipients('You tried to send to recipient(s) that have been marked as inactive. Found inactive addresses: . Inactive recipients are ones that have generated a hard bounce, a spam complaint, or a manual suppression.')

< [ '.' ]