` extract_body_part` can receive empty strings and throws exceptions when it does

[lakinwecker]

Basically https://github.com/pazz/alot/blob/master/alot/db/utils.py#L489 returns an empty string in some circumstances and that is passed into here https://github.com/pazz/alot/blob/master/alot/db/utils.py#L499 which causes an exception. installing the service_identity module did not solve it. Basically I have some emails where the only body part is an application/zip body part