Some IMAP servers return NIL if you BODY.PEEK[TEXT] a messsage with
no body, instead of the more canonical {0}CRLF. Instead of messing with the
parser to deal with that, it makes more sense not to try and fetch empty
bodies. So there are three changes:
* Don't fetch parts when size = 0
* Don't append "null" when there is null body text
* Slight change to attachment handling so size is reported >0
* Unit tests on some of the related lower-level protocol stuff
Bug http://b/issue?id=2160387
Change-Id: Ifb8fb0ed5ce7297908e1ae8d5a02dda5975c4a3c
b6756688b1