Set core asset issuer to null account
Set core asset issuer to null account

No one should be able to interfere with the behavior of core asset, so
codify this.

oxarbitrage/bitshares-core,abitmore/bitshares-2,bigoc/openchain,pmconrad/graphene,cryptonomex/graphene,oxarbitrage/bitshares-core,bigoc/openchain,peertracksinc/muse,bitsuperlab/cpp-play2,abitmore/bitshares-2,abitmore/bitshares-2,oxarbitrage/bitshares-core,bitsuperlab/cpp-play2,abitmore/bitshares-2,bitshares/bitshares-2,oxarbitrage/bitshares-core,bitshares/bitshares-2,peertracksinc/muse,cryptonomex/graphene,pmconrad/graphene,bitsuperlab/cpp-play2,pmconrad/graphene,bigoc/openchain,peertracksinc/muse,bitshares/bitshares-2,bitshares/bitshares-2,pmconrad/graphene,bigoc/openchain,cryptonomex/graphene,peertracksinc/muse