Calculating Business days excluding holidays function with null values

Try this

Txtcher 

let

    fBusDays = (StartDate as nullable date, EndDate as nullable date, HolidayList as list) as nullable number =>

        let

            varBusDays =

                if StartDate = null or EndDate = null then 

                    null

                else 

                    let

                        // Ensure end date is included by adding 1 to the count

                        DateList = List.Dates(StartDate, Duration.Days(EndDate - StartDate) + 1, #duration(1,0,0,0)),

                        RemoveWeekends = List.Select(DateList, each Date.DayOfWeek(_, Day.Monday) < 5),

                        RemoveHolidays = List.RemoveItems(RemoveWeekends, HolidayList),

                        BusDays = List.Count(RemoveHolidays)

                    in

                        if BusDays < 0 then null 

                        else if BusDays = 0 then 1 

                        else BusDays

        in

            varBusDays

in

    fBusDays

You can invoke it like this in a custom column:

fBusDays([Start Date], [End Date], HolidayList)

Make sure HolidayList is a list of dates.