CollectionDates Alert fix

Fix issue if use critical status is set and no threshold is specified.  In this case, the critical status threshold (daily check threshold) should be used rather than the alert threshold.  This will fix the
Cannot insert the value NULL into column 'Threshold' error.
Added ISNULL check just in case.
#1240

DBADashDB/Alert/Procedures/CollectionDatesAlert_Upd.sql

@@ -40,7 +40,7 @@ SELECT 	I.InstanceID,
 		R.Priority,	
 		R.AlertKey,
 		CDS.Reference,
-		CASE WHEN Calc.UseCriticalStatus=1 AND CDS.CriticalThreshold<R.Threshold THEN CDS.CriticalThreshold ELSE R.Threshold END AS Threshold,
+		ISNULL(CASE WHEN Calc.UseCriticalStatus=1 AND (CDS.CriticalThreshold<R.Threshold OR R.Threshold IS NULL) THEN CDS.CriticalThreshold ELSE R.Threshold END,0) AS Threshold,
 		R.RuleID,
 		CDS.SnapshotAge
 FROM Alert.Rules R