DB Engine :: Tracking Changes On 4 Tables

I am using change tracking for tracking changes to an OLTP database for daily data warehouse load. I am tracking changes on 4 tables:

Site, Well, GasEngine, GasField,

I am using an SSIS package for our DW load and on completion, I store the results of the queries above into my own ChangeTracking table. I then use this as the last synced version for each table which I then use for the next DW load. I have the following change tracking settings:

CHANGE_TRACKING = ON
(
CHANGE_RETENTION = 10 DAYS,
        AUTO_CLEANUP = ON
)

Now, the Site and Well tables may not have any changes for a few weeks or months but the GasEngine table will have changes every half hour and the GasField table will have changes every week. I have recently received a NULL value for Site changes query (below) and the SSIS package fails.

Is this due to there being no changes within the 10 days retention period?? What can I do in order to maintain the changes?? Could I maybe update the scripts above to check if the response from the query is NULL and if yes, set the current synced version to the last synced version from my ChangeTracking table?

Also, when the 10 days retention period is up, will the change tracking change version counter continue on from the last change version?