Calls to

api/rhsm-subscriptions/v1/tally/products/rosa/Cores?endi[…]&use_running_totals_format=true&billing_category=prepaid

and similar are throwing NullPointerExceptions because the value of "previous" parameter used in the org.candlepin.subscriptions.tally.filler.TallyReportDataPointAdapter#createDefaultItem method are null.  We had a check for this but the check was actually testing that the "previous.getValue()" call wasn't null instead of checking that "previous" itself wasn't null.