If I have a flow built with some variable that I'm happy for them to be null. Can I still use the flow in PVA as it seems to want a variable set even when the variable is set to optional in the flow input?

If I have a flow built with some variable that I'm happy for them to be null. Can I still use the flow in PVA as it seems to want a variable set even when the variable is set to optional in the flow input?