I had this same issue a couple weeks back. I was looking for a node to put in my scene that was essentially empty. Group node was my first thought, but I had no need for a transform so I kept looking.
There lots of node types in Maya, but I was looking for one that had very few other attributes, so when I had it open in the AE, it would be mostly empty aside from my extra attributes.
Unknown fits this bill, but it made me a bit uncomfortable to use a node that was primarily for handling undefined nodes. It is hard to put your trust in the unknown.
I ended up going with with a materialInfo node, which is a very empty node type and seemed to make sense as my work was shader related.
Hope this helps.