Backup missing deleted users information

Regression of MDL-29602, while testing MDL-30192, this was discovered.

Before 2.2, accesslib->get_context_instance() always returned one context for any user. Since some 2.2, deleted users don't get any context instance anymore.

This was causing, in backup, to miss some user's information completely, leading to errors both in restore and also after restore, trying to access to information about those users.

So this is about to fix backup 2.2 so it will continue including deleted users as was done before.

Once implemented, the tests @ MDL-30192 for 2.2 should be enough to test that this is working.