Add skip_serializing_none feature

[Selyatin]

Closes #411 and #262 :)

[tomhoule]

nice, thanks for the PR!

I think there are two points to address before we can merge this:

• We should change the way the attribute value is specified, expecting true as a string is not a good API. Maybe something like #[graphql(schema_path = "...", query_path = "...", skip_serializing_none)] or #[graphql(schema_path = "...", query_path = "...", skip_serializing_none = true)]? (I prefer the first one, and it should be doable)

• We should have at least one end-to-end test that checks that the flag works by actually serializing an input (there are a bunch of examples in https://github.com/graphql-rust/graphql-client/tree/main/graphql_client/tests) — asserting that the generated code contains a specific substring sounds a bit brittle.

[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?