if (data.accountNumber === "null") {
const body = JSON.parse(pm.request.body.raw);
body.accountNumber = null
pm.request.body.raw = body
}
This needs to go in the pre-request script.
data is a special variable which contains the data for the current iteration. So you should be able to have conditional statement checking for the accountNumber === “null”. The string.
You can then parse the current request body, update the account number to be the correct type of null, and then resave the body which will then be used in the request.