VARCHAR NULL storage I read somewhere online and in a Microsoft Press book there is a field declaration attribute that can be used to indicate NULL VARCHAR values should not consume space at the leaf-level instead of consuming a few characters per leaf.

Could someone help me identify where that attribute is? 

I cant remember what the Attribute keyword is but I remember seeing it because of the value of saving the empty 2 bytes and make it a 0 byte storage for no data.
I believe you are talking about SPARSE columns.