You can use this expression: $JsonStr != null && $JsonStr != “” ? $JsonStr .replaceAll(“\n”,“”) : $JsonStr
Explanation: This will first check if the JsonStr is null or empty, if it’s not then it’ll do the replacing else it’ll just put input jsonStr as output. In your case where JsonStr is null it’ll show output null
I mentioned $JsonStr because you mentioned it in the sample data that you shared in comment.
$Jsonstr is the name of the input string that is coming from upstream snaps.
Now if you want to check if a specific field is there or not, you can use the below.
$.hasPath(‘$JsonStr’) ? ($JsonStr != null && $JsonStr != “” ? $JsonStr .replaceAll(“\n”,“”) : $JsonStr) : “”
Let me know if it works.