Calendar include blanks

Hi,

I have a start & end date with some blanks in the rows which I would like included, I need to handle them differently in the visual layer. However the DAX calendar function says blanks are not allowed.

I tried adding a IF(ISBLANK()) but this returns a error saying end date must be larger than start date, so I'm out of ideas now.

Delivery Days = 
COUNTROWS (
    FILTER (
        ADDCOLUMNS (
            CALENDAR ( 
                'Toll_Shipments'[Pick Up Date],
                Toll_Shipments[Actual Delivered Date]
                  ),
            "Is Weekday",WEEKDAY([date],2)<6
            ,"Is Holiday", CONTAINS('Holiday Dates','Holiday Dates'[Dates],[Date] )
        ),
        [Is Holiday] = FALSE() 
           && [Is Weekday] = TRUE()
    )
)

 Thanks for any ideas or assistance.