Hi,
It would be helpful if you can specify what input you are trying to pass to the input parameter.
Based on what I understood, as a workaround, you can create a calculated column on REGION field like
IF(ISNULL("REGION"),'',"REGION")
This would convert the null values into a blank sting ('') and then you can do the comparison.
You can also try to use the ISNULL("REGION") in your filter condition if that helps.
Regards,
Piyush