I have created the above value rule in the ribbon workbench 2016.  If the Opportunity Scope = "Product" and Document Status is not equal to "Ready To Export" or "Exported To GP", the Closed as Won button will be hidden.  However, there is one more wrinkle I'd like to add.  If the Opportunity Scope = "Product" and the Document Status is blank, I want the Closed as Won Button to be hidden as well.  For some reason this is not working with the workflow above.  When the Document Status field is blank, the button still shows.  I tried adding another "Or" step that contains a Value rule, with "new_documentstatus" as the Field Name.  I then left the Value field blank, and set the InvertResult to True, but that did not work.  I also tried setting the value rule to "null", but that did not work either.  How can I fix this?

Thanks!