Add skip_serializing_none feature

[Selyatin]

Closes #411 and #262 :)

[Selyatin]

@tomhoule requested changes were made, let me know if the test looks good.

[Nohac]

Sorry for bumping this closed PR, but I'm really struggling to get this to work. I've added skip_serializing_none to the struct as stated in the README, but doing so still leaves me with null values after serialization.

My first guess was that this hasn't been released yet, so I switch over to the main branch in Cargo.toml, but that changed nothing. Here's the code I'm using:

#[derive(GraphQLQuery)]
#[graphql(
    schema_path = "graphql/schema.graphql",
    query_path = "graphql/insert_mutation.graphql",
    response_derives = "Debug",
    skip_serializing_none
)]
pub struct InsertMutation;

let vars = Variables {
    // .. omitted
    user_id: None,
    // .. omitted
};

let q = crate::graphql::InsertMutation::build_query(vars);

let skipped = serde_json::to_string_pretty(&q).unwrap();
println!("{skipped}");

Output:

{
  "variables": {
    // .. omitted
    "user_id": null,
    // .. omitted
  },
  ...
}

Is there something obvious I'm missing here to make this work, or is this something that warrants it's own issue?