Properly manage the case config is None
For example: 

https://datasets-preview.huggingface.tech/splits?dataset=sent_comp&config=null returns

~~~json
{
    "dataset": "sent_comp",
    "config": "null",
    "splits": [
        "validation",
        "train"
    ]
}
~~~

this should have errored since there is no `"null"` config (it's `null`).

https://datasets-preview.huggingface.tech/splits?dataset=sent_comp&config= returns

~~~json
The split names could not be parsed from the dataset config.
~~~

https://datasets-preview.huggingface.tech/splits?dataset=sent_comp returns

~~~json
{
    "dataset": "sent_comp",
    "config": null,
    "splits": [
        "validation",
        "train"
    ]
}
~~~

---

As a reference for the same dataset https://datasets-preview.huggingface.tech/configs?dataset=sent_comp returns

~~~json
{
    "dataset": "sent_comp",
    "configs": [
        null
    ]
}
~~~

and https://datasets-preview.huggingface.tech/info?dataset=sent_comp returns

~~~json
{
    "dataset": "sent_comp",
    "info": {
        "default": {
            ...
            "builder_name": "sent_comp",
            "config_name": "default",
            ...
            "splits": {
                "validation": {
                    "name": "validation",
                    "num_bytes": 55823979,
                    "num_examples": 10000,
                    "dataset_name": "sent_comp"
                },
                "train": {
                    "name": "train",
                    "num_bytes": 1135684803,
                    "num_examples": 200000,
                    "dataset_name": "sent_comp"
                }
            },
            ...
        }
    }
}
~~~

Properly manage the case config is None: For example: 

https://datasets-preview.huggingface.tech/splits?dataset=sent_comp&config=null returns

~~~json
{
    "dataset": "sent_comp",
    "config": "null",
    "splits": [
        "validation",
        "train"
    ]
}
~~~

this should have errored since there is no `"null"` config (it's `null`).

https://datasets-preview.huggingface.tech/splits?dataset=sent_comp&config= returns

~~~json
The split names could not be parsed from the dataset config.
~~~

https://datasets-preview.huggingface.tech/splits?dataset=sent_comp returns

~~~json
{
    "dataset": "sent_comp",
    "config": null,
    "splits": [
        "validation",
        "train"
    ]
}
~~~

---

As a reference for the same dataset https://datasets-preview.huggingface.tech/configs?dataset=sent_comp returns

~~~json
{
    "dataset": "sent_comp",
    "configs": [
        null
    ]
}
~~~

and https://datasets-preview.huggingface.tech/info?dataset=sent_comp returns

~~~json
{
    "dataset": "sent_comp",
    "info": {
        "default": {
            ...
            "builder_name": "sent_comp",
            "config_name": "default",
            ...
            "splits": {
                "validation": {
                    "name": "validation",
                    "num_bytes": 55823979,
                    "num_examples": 10000,
                    "dataset_name": "sent_comp"
                },
                "train": {
                    "name": "train",
                    "num_bytes": 1135684803,
                    "num_examples": 200000,
                    "dataset_name": "sent_comp"
                }
            },
            ...
        }
    }
}
~~~
