net_processing: Move pindexBestKnownBlock and pindexBestHeader NULL usage to nullptr.
net_processing: Move pindexBestKnownBlock and pindexBestHeader NULL usage to nullptr.

PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX