SQL Server NULL Index
These "indexes" are actually not indexes at all. They're called heaps, which is an internal structure that holds the table data without using any ordering mechanism like an index does.

Since there's no order, there's nothing to do as far as reorganizing or rebuilding goes, so these can be safely ignored as far as consideration for index maintenance. I would recommend using a 3rd-party solution for index maintenance though.

If you're new to index structures in SQL Server, I have a video here that explains them.