Changed contrib report query to handle case of no contacts for account list

I hadn't commented on the pull request before, but thanks Justin for catching the bug in the contribution report query to handle cases where a contact is in multiple account lists in which case they may not appear on the contribution report.

This changes the inner query for `distinct_contact_donor_accounts` to handle the case where there are no contacts for an `account_list` (which would otherwise cause a SQL syntax error) and also uses a single query.

Does this look right to you @jbirdjavi, or any alternate suggestions?