Fix deserialization error on missing file retention

[st3iny]

Fix https://todo.sr.ht/~rjframe/b2-client/8

The field FileRetention::value might be null in some cases. Please refer to my test program in the linked issue to reproduce the bug.

Sample response from b2

{
  "files": [
    {
      "accountId": "redacted",
      "action": "upload",
      "bucketId": "redacted",
      "contentLength": 286219833,
      "contentMd5": null,
      "contentSha1": "none",
      "contentType": "aplication/octet-stream",
      "fileId": "redacted",
      "fileInfo": {
        "src_last_modified_millis": "1668455229615",
        "large_file_sha1": "90750400ac8f18c97ae284b52fa1d9651e752790"
      },
      "fileName": "redacted",
      "fileRetention": {
        "isClientAuthorizedToRead": false,
        "value": null
      },
      "legalHold": {
        "isClientAuthorizedToRead": false,
        "value": null
      },
      "serverSideEncryption": {
        "algorithm": null,
        "mode": null
      },
      "uploadTimestamp": 1668455297058
    }
  ],
  "nextFileName": null
}