notifications: fix notification expiry when no user is associated

notifications: fix notification expiry when no user is associated

It's possible that an EmailConfirmation object will have no associated
user (eg, for email opt-out, which does not require a user object). In
this case, we will see a NULL value for EmailConfirmation.user_id.

However, having a NULL value appear in a SQL 'IN' clause will match
every value. This means that once one of these null-user
EmailConfirmations is present, we will never expire any non-active user
accounts.

This change adds a filter for a valid user_id when we query for active
EmailConfirmation objects. This means we'll have a valid values set to
use in the pending_confs set.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
[dja: fix pep8 issue]
Signed-off-by: Daniel Axtens <dja@axtens.net>