Disallow empty GraphQLBatchRequests

[tyranron]

Is your feature request related to a problem? Please describe.
At the moment, it's possible to deserialize GraphQLBatchRequest as an empty Vec<GraphQLRequest>. The downstream implementations (like juniper_warp) rely transparently on that, executing empty batch and returning empty response.

I'm unsure this is OK. Maybe, it's better to reject empty batches at deserialization level?

Describe the solution you'd like
The one was implemented once in early juniper_actix iterations:

enum GraphQLBatchRequest<S = DefaultScalarValue>
where
    S: ScalarValue,
{
    Single(GraphQLRequest<S>),
    #[serde(deserialize_with = "deserialize_non_empty_vec")]
    Batch(Vec<GraphQLRequest<S>>),
}

fn deserialize_non_empty_vec<'de, D, T>(
    deserializer: D,
) -> Result<Vec<T>, D::Error>
where
    D: de::Deserializer<'de>,
    T: Deserialize<'de>,
{
    let v = Vec::<T>::deserialize(deserializer)?;
    if v.is_empty() {
        Err(de::Error::invalid_length(0, &"a positive integer"))
    } else {
        Ok(v)
    }
}

Describe alternatives you've considered
Leave it "as is". But seems to be more error-prone for frontends.

---

If it something desirable, I'll submit the PR shortly.

[LegNeato]

Yeah, I think we should reject empty batch requests as it almost certainly points to a bug on the client side.