The field’s Default formula ONLY executes if the field doesn’t exist in the document which in practice means that its value is “”
So, your code will never execute because even if you get the @If statements correct, you wish to preserve the “” if that is the value of the field and this will be the case for new documents.
I suppose that you want to do this in the Translation formula but in that case, you have to be careful to not modify the field by adding the “.enc” each time the document refreshes.
Maybe you should describe what you are trying to achieve.