When you use Rest receiver, it's common to use array structures.
In some cases the array has only one position. In this case the transformation has an issue. The destination structure don't recognize the array.
The trivial solution for this is to add dummy element to the array. you need to design this with the receiver syste team. They must to know the last position is dummy, you can use aspecial value, like id=-1, or position = 999. Values that are not valid from business perspective.
Let me show a little example
You can see element PositionArray, the last iteration has a position 9999