That is an interesting questions. Currently, there is no easy way to test if a variable is present or not. There is an open issue to handle this case: https://github.com/zeebe-io/zeebe/issues/4212
I recommend to initialize all variable with null. In a FEEL expression, you can check if a variable is null (e.g. if(x = null) then "default" else x).
Alternatively, you could also check if the variable has a specific type. For example: if(x instance of string) then x "default"