rowaggregation asserts for agg on char(5)

We noticed this when testing moda with char type.

Row aggregation will assert if the length of the field isn't = 1,2,4 or 8. The reason being is that it tries to set the accumulator to NULL during start of processing. For small char fields, they're stored as unsigned integers. With a size of 5 (or3,6,7) the function asserts because we don't have predefined values for NULL for these sizes.

See rowaggregation::makeAggFieldsNull()