parser: Correct empty email value check

parser: Correct empty email value check

The check for empty emails in 'find_author' checked for None, but it
was not possible to ever return None unless the 'From:' header was
missing altogether. Seeing as, per RFC822 and its revisions, this is
not possible, check for the empty string instead.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Andy Doan <andy.doan@linaro.org>