SSIS Data Flow Task Excel to SQL table NULL value will not work for small INT datatype
My original answer was working (changing int to nvarchar) but I ran into another column which contained dates in the excel source file where some cells contained the string "NULL" (I did not want to have dates in a NVARCHAR column.) When SSIS got to this date column it was generating an error because it could not convert the string "NULL" to a date. It was not properly reading the "NULL" as a NULL value, instead it was reading it as a string. The way I was able to resolve this issue is by adding a derived column component to the package that replaced the date field by using the following expression

(DT_WSTR,255)Auth_from_date == "NULL" ? NULL(DT_WSTR,50) : (DT_WSTR,255)Auth_from_date


this is just an IF statement that will change the string "NULL" to an actual NULL value. The date is then passed thru a data conversion component and converted into DATETIME format.