[Matrix]: avoid adding empty vectors to groupby result of data frame.
[Matrix]: avoid adding empty vectors to groupby result of data frame.

flashxio/FlashX,icoming/FlashGraph,zheng-da/FlashX,icoming/FlashGraph,zheng-da/FlashX,zheng-da/FlashX,flashxio/FlashX,icoming/FlashX,flashxio/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashGraph,zheng-da/FlashX,icoming/FlashX,flashxio/FlashX,zheng-da/FlashX,icoming/FlashGraph,flashxio/FlashX,flashxio/FlashX,icoming/FlashX,icoming/FlashX