Fixed bug: DB migrations won't run if dbPrefix = ''.

Also fixed but not in this PR (because they're in the premium version):

        app/protected/migrations/m140401_230921_add_configs_to_user_setting_table.php
        app/protected/migrations/m140617_215248_add_monitor_sent_mail_to_user_setting_table.php
        app/protected/migrations/m140618_202801_add_mark_as_read_to_folders.php
        app/protected/migrations/m140618_202805_add_mark_as_read_to_senders.php
        app/protected/migrations/m140623_225143_add_expire_to_folder_table.php
        app/protected/migrations/m140623_225737_add_challenge_name_to_account_table.php
        app/protected/migrations/m140623_225946_create_sent_message_table.php
        app/protected/migrations/m140623_230255_add_in_reply_to_to_message_table.php
        app/protected/migrations/m140626_002654_extend_user_setting_table_for_training_emails.php
        app/protected/migrations/m140630_171231_extend_user_setting_table_for_unanswered_digest.php