Fix users selected queries not returning any data

Previously adding the users entry in the database.rows_count.selected_queries configuration would return no new data. This was happening because there was a WHERE user_type IS NULL query that caused no rows to match.

Since GitLab 16.0, all human users now have a user_type of 0 instead of NULL: gitlab-org/gitlab!115849 (merged).

Adjust the query to match the schema.

Relates to #109 (closed)