Avoid database lookup and update when new ram is not issued with every block.

[greg7mdp]

Add an early return if _gstate2.new_ram_per_block == 0 which is the case now:

See also in reference-contracts

https://github.com/eosnetworkfoundation/eos-system-contracts/blob/8ecd1ac6d312085279cafc9c1a5ade6affc886da/contracts/eosio.system/src/eosio.system.cpp#L93-L109

[bhazzard]

Also make these changes in AntelopeIO/reference-contracts