Wall: fixing WikiaBot's posts on Empty's message wall

There are two fixes here, both related to WikiaBot's posts mentioned in bug 95249. The first issue was happening on walls without any messages - in this case the getWhere() method was returning an SQL condition asking for messages with parent_page_id set to 0, which resulted in showing broken messages on those walls.
The second fix addresses the problem with Special:Contributions for WikiaBot, which was showing a ton of posts on Empty's wall. In fact those entries are related to the fact that WikiaBot is creating message wall pages in namespace 1200 when the first item is posted. So after this fix those contributions will be propertly showed as wall creations.