For GenericUDAFs, iterate function is called despite empty table

For empty input tables, the iterate function in GenericUDAF is called once with a null input for empty table. This makes it impossible to differentiate the empty table case for the case where there the table has one row with a null in the desired column.

This happens only when the genericUDAFcall is made with a column name specified and not when the '*' wild card in used.