fix(utxorpc): correctly handle empty datum fields in ReadUtxos response

[verbotenj]

Fixes #251

// Hash tx
base64: eRMJtrD6zICz+olugwmZ577zIepSefa+2+EnnuHp1K4=
// Hex value of base64 Hash tx
hex: 791309b6b0facc80b3fa896e830999e7bef321ea5279f6bedbe1279ee1e9d4ae

Returns expected

grpcurl -plaintext -d '{"keys": [{"hash": "eRMJtrD6zICz+olugwmZ577zIepSefa+2+EnnuHp1K4="}]}' localhost:9090 utxorpc.v1alpha.query.QueryService.ReadUtxos
{
  "items": [
    {
      "nativeBytes": "glg5AJ1hk0QlxWy+JWCbQgZaNWvZunvonoqgIo6R7aam1QLMzKKDJ/IuoQPXLLQ9gK4z7mgkNrZADfm0GgCYloA=",
      "txoRef": {
        "hash": "eRMJtrD6zICz+olugwmZ577zIepSefa+2+EnnuHp1K4="
      },
      "cardano": {
        "address": "AJ1hk0QlxWy+JWCbQgZaNWvZunvonoqgIo6R7aam1QLMzKKDJ/IuoQPXLLQ9gK4z7mgkNrZADfm0",
        "coin": "10000000"
      }
    }
  ],
  "ledgerTip": {
    "slot": "75089835",
    "hash": "dv2WlIwT2BtpUL6HcSgUK4z32wtJ7m3LJnPjWv/YOuk="
  }
}

cardano-cli conway query utxo \
--tx-in 791309b6b0facc80b3fa896e830999e7bef321ea5279f6bedbe1279ee1e9d4ae#0 \
--testnet-magic 1 \
--output-text
                           TxHash                                 TxIx        Amount
--------------------------------------------------------------------------------------
791309b6b0facc80b3fa896e830999e7bef321ea5279f6bedbe1279ee1e9d4ae     0        10000000 lovelace + TxOutDatumNone