If you want to set a property with a null value you should proceed as follows:
- Go to "Custom Actions" page and create a "Set installer property" custom action with sequence under "Wizard Dialogs Stage" -> "Begin" standard action.
- Set "Property" field to your property name (i. e. MYPROP).
- Set "Formatted" field to:
- Code: Select all
Also, you can obtain the same behavior if you define your property with no default value from User Interface (i.e. create an edit box control and set its property with no default value from "Properties" right pane).
If you have any questions let me know.
All the best,