ImageServing: prevent queries by page_id = 0

@Wikia/platform-team 

Avoid the following queries:

```sql
SQL SELECT /* ImageServing::getImages Bloodsucker616 */  page_namespace as ns,page_title as title,page_id as id  FROM `page`  WHERE page_id = '0'
```
I think you can drop the comment :). It's clear what's happening here.