Store migrator sets base tx checksum if migrating empty store

since there's this convention that txId=BASE_TX_ID has checksum
BASE_TX_CHECKSUM. This logic exists in some places and StoreMigrator
now follows that convention as well to avoid checksum mismatches.
Merge pull request #3677 from tinwelint/2.2-base-tx-checksum-empty-migration

Store migrator sets base tx checksum if migrating empty store