Fix return type of message without a preferred body

[johslarsen]

The get_body(...) call (and hence the nominal return of the get_body_part(...) method) returns a email.message.EmailMessage. So if there is no body return an empty such class instead of a string, because the callers expects that it behaves like a message. E.g.:

  File "alot/db/utils.py", line 499, in extract_body_part
    **{'field_key': 'view'} if body_part.get_content_type() == 'text/plain'
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'get_content_type'

Someone sent me a message where the body was some encapsulated message. I still have not gotten it rendered properly, but showing up as an empty message is at least better than crashing alot

[pazz]

I agree with this. The code is simple and fixes a known issue.
Lont term it would perhaps be good to recurr in such situations but better displaying an empty mail than going belly up

[pazz]

Thanks for your contribution :)