You want your columnA to be:
That is accomplished in the schema source of the mapping. If that is an XML, then you can add
minOccurs="1" nillable="true"
to the elements definition.
If it is JSON, I think it is not possible, hope to stand corrected.
Worst case you can use a microflow to create the required outcome.
Thank you very much.
It is JSON .I understand that.