make invalid_hereafter option<String> from String

[jackdfraser]

Summary

updated invalid_hereafter to be Option<String> from String. Added a test using the data that made the sdk fail.

Background

the block frost api is returning null for the invalid_hereafter field for tx_hash 78e5821367cd4e2fbfce4d32c3ecededd388f13118f06b959015c2aad19b5cd8.

reference javascript, dotnet sdks with invalid_hereafter as nullable

Error Message

{
  "url": "https://cardano-mainnet.blockfrost.io/api/v0/txs/78e5821367cd4e2fbfce4d32c3ecededd388f13118f06b959015c2aad19b5cd8",
  "reason": "Error(\"invalid type: null, expected a string\", line: 1, column: 471)",
  "text": {
    "hash": "78e5821367cd4e2fbfce4d32c3ecededd388f13118f06b959015c2aad19b5cd8",
    "block": "bb4003096f11eaaca11d3705db62637f311f93671fcb4d4b3cf8749a90ea4b74",
    "block_height": 6352717,
    "block_time": 1633861069,
    "slot": 42294778,
    "index": 9,
    "output_amount": [
      {
        "unit": "lovelace",
        "quantity": 4828735
      },
      {
        "unit": "29d222ce763455e3d7a09a665ce554f00ac89d2e99a1a83d267170c64d494e",
        "quantity": 25000000000000
      }
    ],
    "fees": 171265,
    "deposit": 0,
    "size": 357,
    "invalid_before": null,
    "invalid_hereafter": null,
    "utxo_count": 3,
    "withdrawal_count": 0,
    "mir_cert_count": 0,
    "delegation_count": 0,
    "stake_cert_count": 0,
    "pool_update_count": 0,
    "pool_retire_count": 0,
    "asset_mint_or_burn_count": 1,
    "redeemer_count": 0,
    "valid_contract": true
  }
}

[marcospb19]

Thanks for your contribution! Looks good :)