format error: `failed` (and `pending`) should not have `split` field for config-size


https://datasets-server.huggingface.co/size?dataset=stas/openwebtext-10k

```json
{
  "size": {
    "dataset": {
      "dataset": "stas/openwebtext-10k",
      "num_bytes_original_files": 0,
      "num_bytes_parquet_files": 0,
      "num_bytes_memory": 0,
      "num_rows": 0
    },
    "configs": [],
    "splits": []
  },
  "pending": [],
  "failed": [
    {
      "kind": "config-size",
      "dataset": "stas/openwebtext-10k",
      "config": "plain_text",
      "split": null
    }
  ]
}
```
format error: `failed` (and `pending`) should not have `split` field for config-size: 

https://datasets-server.huggingface.co/size?dataset=stas/openwebtext-10k

```json
{
  "size": {
    "dataset": {
      "dataset": "stas/openwebtext-10k",
      "num_bytes_original_files": 0,
      "num_bytes_parquet_files": 0,
      "num_bytes_memory": 0,
      "num_rows": 0
    },
    "configs": [],
    "splits": []
  },
  "pending": [],
  "failed": [
    {
      "kind": "config-size",
      "dataset": "stas/openwebtext-10k",
      "config": "plain_text",
      "split": null
    }
  ]
}
```