Warnings in user_node_load() for deleted users

Latest patch is still good, back to RTBC.

I think there are few important comments here, like #8 and #10, so let's discuss this a bit more. This seems to be caused by inconsistent database as normally it should not be possible to delete a user while the UID is left in the node table.

This patch seems like an easy workaround, but it can probably hide more problems - there can be other parts in code where the existence of that user is expected. I think the cleaner solution will be something like David proposed in #10, for example to update all nodes where UID is not in users table to 0 (anonymous user). However I can imagine this can have a pretty big impact on some sites with lot of content referencing non-existing users. On the other hand, I would not expect to have an empty username in any case (I would expect either Anonymous, or user name). So moving this to Needs Review for some additional discussion. Thanks.