Sorry to say there is no such Api or events fit for this case based on the current structure. The reason is no matter what the column type is, in the last end, its value will be converted to the string, and the null-able value would be converted to the empty string as the feature ColumnValue.
I am thinking is it possible for us to replace the Null-able value with some special characters like "N/A" in the original shape file and once we detect the "N/A", we can treat this column as Null. But I also worry the "N/A" would not be compatible for every data type in shape file, So, this way is just a simple method and need more works to finish.
Hope it helps and please feel free to let us know if any questions.