cognito: Fix nullable cognito pre-token-gen event ClaimsOverrideDetails

[sebboer]

The actual incoming event for the Cognito pre-token-gen should have the claimsOverrideDetails field as optional.
This is what the incoming event looks like:

{
  "version": "1",
  "triggerSource": "PreTokenGen",
  "region": "region",
  "userPoolId": "userPoolId",
  "userName": "userName",
  "callerContext": {
    "awsSdkVersion": "calling aws sdk with version",
    "clientId": "apps client id"
  },
  "request": {
    "userAttributes": {
      "email": "email",
      "phone_number": "phone_number"
    },
    "groupConfiguration": {
      "groupsToOverride": ["group-A", "group-B", "group-C"],
      "iamRolesToOverride": ["arn:aws:iam::XXXXXXXXXXXX:role/sns_callerA", "arn:aws:iam::XXXXXXXXX:role/sns_callerB", "arn:aws:iam::XXXXXXXXXX:role/sns_callerC"],
      "preferredRole": "arn:aws:iam::XXXXXXXXXXX:role/sns_caller"
    },
    "clientMetadata": {
      "exampleMetadataKey": "example metadata value"
    }
  },
  "response": {
    "claimsOverrideDetails": null
  }
}

The events for cognito are not really well documented and differ from the actual events.