GitLab

* * Workaround for graph data saving/loading compatibility problem in Column class.  There may be more places in DGL with the same issue, due to using Python serialization, instead of a more cohesive, comprehensive strategy.  This is just a local fix.

* Add checking for non-empty states

* Add unit test

* Handle the case of columns without storage
Co-authored-by: ndickson <ndickson@nvidia.com>
Co-authored-by: Xin Yao <xiny@nvidia.com>