I don´t really get it why you want to hide the null values but it might work in script
if(Len(Trim(Field1))=0, "Here you can add you´re favourite expresion","else expression") as FieldName
Set NullValue = 'you´re expression for null values';
or without script (but this doesn´t work all the time if you import it from excel)
right click on your pivot, properties, presentation , then you can define how null values should be displayed.
BUT i think you can´t remove them completely from a pivot-table.