You can choose the empty function in Power Automate to check if a specific value is null or not.
This expression checks if the value of the specific column is empty or null. If it is not empty or not null, and if you want to check if it is null change the condition to be true instead of false
@equals(empty(triggerOutputs()?['body/SupervisorEmail']), false) this will check if the SupervisorEmail is not null