Filter for non-empty - adjustment.

This adjustment allows more flexibility when we need to retrieve entry ids for content. The default implementation is for fields whose content resides in the content field of dataform contents. In this case we retrieve the entry ids from all field content records we can find. In some cases however, we may need a custom query. For instance, if we want to fetch all entries which have a type set, or all entries which don't have a type set. Then we need to query on the type column of the dataform_entries table.