How can a configure a null value using RPA?

Summary:

We have a use case where an integration is ingesting a file. The Supplier Number field is not allows populated in the file. The integration used to send 'None' to replace the null value, but this has since been fixed by introducing code in the integration.

I'm wondering how I can create a Switch condition that if the Supplier Number exist, enter text in the Supplier Number field. If Supplier Number does not exist, go to condition 2 and don't try and populate anything in the Supplier Number field.

I've tried creating an expression (${$VARIABLE.Current_Invoice[Supplier Number]} = null) for the variable but I continue to get a Validation error with each try.