Making the correct answer blank for a test question

A lot depends on how you give feedback on the question. What I would do is have an action either on the Check My Answer button or an On Hide action at the page level that checked for an empty question variable and if so, set it to a single blank:

On Hide on the page or On Click on a Check My Answer button
Action: Modify Variable
Target: Question variable xxxx
Value: (enter a single blank)

And when creating the question make the correct answer a single blank.