Moto allows empty ExpressionAttributeValues in transaction put item

[regoawt]

Hi,

Transact write items fails with An error occurred (ValidationException) when calling the TransactWriteItems operation: ExpressionAttributeValues must not be empty when making the request against DDB, but moto allows it:

[
  {
    "Update": {
      "Key": { "pk": { "S": "globals" }, "sk": { "S": "globals" } },
      "UpdateExpression": "SET #0 = :0",
      "ExpressionAttributeNames": { "#0": "tenant_count" },
      "ExpressionAttributeValues": { ":0": { "N": "1" } },
      "TableName": "Table1"
    }
  },
  {
    "Put": {
      "Item": { "pk": { "S": "tenant#0000001-tenant#0000001" }, "sk": { "S": "tenant#0000001" } },
      "ConditionExpression": "attribute_not_exists(#n0)",
      "TableName": "Table1",
      "ExpressionAttributeNames": { "#n0": "pk" },
      "ExpressionAttributeValues": {}
    }
  }
]

The Update operation works fine but the Put should raise an error.