juniper_graphql_ws: serde deserialization error with null values

[lightdiscord]

Describe the bug
When a client start a subscription it sends a payload which can contain optional variables. Currently, serde default field attribute is being used.
It works if the key is not present or if it is an empty object ({}) but yield a serde error if it's null (invalid type: null, expected a map).

To Reproduce

• Launch the actix_subscriptions example.
• Launch a client (graphiql for example) and send a payload with "variables": null (with graphiql it should send null if no variable are specified)

Expected behavior
Treating the field as if it were absent.

Additional context
This kind of problem was already discussed on this issue.

Potential fix
One way to solve the issue is to use the default attribute and deserialize_with for example:

fn unwrap_or_default<'de, D, T>(deserializer: D) -> Result<T, D::Error>
where
    D: Deserializer<'de>,
    T: Deserialize<'de> + Default
{
    Ok(Option::<T>::deserialize(deserializer)?.unwrap_or_default())
}

#[derive(Debug, Deserialize, PartialEq)]
#[serde(bound(deserialize = "S: ScalarValue"))]
#[serde(rename_all = "camelCase")]
pub struct StartPayload<S: ScalarValue> {
    pub query: String,
    #[serde(default, deserialize_with = "unwrap_or_default")]
    pub variables: Variables<S>,
    pub operation_name: Option<String>,
}

[LegNeato]

Sounds great, thank you for the report and the suggested fix! Would you be willing to put a PR up?

[lightdiscord]

No problem. I'll open a PR.