Spam displayed in activity stream

[yoavf]

There's a specific scenario that will cause spam caught by Akismet to be displayed in the site activity stream:

1. There are users in the system without an email address
2. Email is not required for comments, and so spambots will send email-less spam comments

The comments will be displayed in the activity stream as assigned to the first user in the db without an email address.

If that user is deleted, subsequent spam comments will be assigned to the next user in the db without an email address.

[johnjamesjacoby]

(In [3209]) Fixes #2590