Handle undefined timestamp for zkEVM batch

Relates to https://github.com/blockscout/blockscout/pull/9563

The `timestamp` item now can be null in `/api/v2/zkevm/batches`, `/api/v2/zkevm/batches/:batch_number`, `/api/v2/transactions/zkevm-batch/:batch_number`. In this case we need to display `Undefined` in the `Age` column on UI.