Skip None fields when serializing generated input objects

[achitojha]

Issue : https://github.com/shop/issues-shopifyvm/issues/318.

This updates generated input-object serialization so generated Option<T> fields that are None are omitted instead of serialized as explicit null values.

Tests

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings

[Copilot]

Pull request overview

This PR changes the generated Serialize implementation for GraphQL input objects so that optional fields (those generated as Option<T>) are omitted from the serialized output when None, rather than being written as explicit null values. Previously every field was written unconditionally, which led to unwanted explicit nulls in the wasm-API output buffer.

Changes:

• In ShopifyFunctionCodeGenerator::additional_impls_for_input_object, split field emission by is_required(): required fields are always written, optional fields are wrapped in an if let Some(value) = ... and only written when present.
• Compute the object's field count dynamically as num_required_fields + Σ usize::from(self.<opt>.is_some()) so write_object receives the correct size.
• Add a SerializationProbe input to the example schema and three tests covering omission of None optional fields, inclusion of Some fields, and @oneOf input object serialization.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
shopify_function_macro/src/lib.rs	Generates conditional emission of optional input-object fields and computes the field count from required + present-optional counts.
example_with_targets/src/tests.rs	Adds tests verifying that None fields are omitted, Some fields are included, and @oneOf variants serialize correctly.
example_with_targets/schema.graphql	Adds SerializationProbe input with a mix of optional, defaulted, and required fields to support the new tests.

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[adampetro]

Can we do some validation on how this affects instruction count in the worst case (when we don't skip serialization but have some extra checks to do)?