patman: Fix error when the email blacklist is missing

This section of the settings file may be missing. Handle that gracefully
rather than emitting an error.

Also update patman to write this section when a new settings file is
created.

Fixes: e11aa602 (patman: add support for omitting bouncing addresses)

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Chris Packham <judge.pckham@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>