Import.pm: Deal with potentially missing From and Sender headers
Use ||= '' to ensure that if the From or Sender header is not present
the code sees an empty string and instead of undefined.
I had some email messages with a From field without an @ (because the
sender was local) and without a Sender which were causing errors when
imported. I think this was bad enough that the email messages were
failing to be imported.
Signed-off-by: Eric Biederamn <ebiederm@xmission.com>