Running generate_optimal_prompt when the variables can hold a null value cause it to crash

[MarianoCornejo]

I was trying to generate a prompt where my sentence could generate a json object , but a possible value for the json is null, so when is generating the test cases it looks like is using None and this can be put inside the var_dict, the problem is null is a valid output for the json and replacing it will empty string will only make the final output to use empty instead of null