Let transaction_cids.dst be null

[i-norden]

dst address is null in the case of a contract deployment, right now we are using a "0" representation of that 0x00000000000000000000 which is problematic for two reasons

1. It takes up more space than NULL
2. Sometimes the dst actually is 0x00000000000000000000 e.g. in the case of a burn transaction, and we want to be able to differentiate this

First,
UPDATE eth.transaction_cids SET dst = NULL WHERE dst = '0x00000000000000000000';

Then,
ALTER eth.transaction_cids ALTER COLUMN dst SET NOT NULL;