In order to check if a variable is "NULL" we need to perform the following actions:


First we need to use the action "Convert Json To Custom Object" and in the "JSON" box type the following:


{
key:null
}


Afterwards we can use the "Set Variable" and set the value of the variable to be the same as the "key" in the Json we just created. This will give us a variable that we know is "NULL" so that we can compare it to our variable using an "IF" action.


In the "IF" action we can use the "Contains" comparison to check if our variable is NULL.


In the code shown in the screenshot below, the process will enter the "If" block if %VariableWeWantToCheck" is NULL.