BlockchainLMDB: Replace remaining txn pointer NULLs with nullptr
BlockchainLMDB: Replace remaining txn pointer NULLs with nullptr

For consistency.

eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,eiabea/bitmonero,ranok/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,ranok/bitmonero