fix(cpp): silence unused-parameter warnings for empty objects

[schani]

What was broken

For a JSON Schema empty object ({"type": "object", "additionalProperties": false}), quicktype's C++ target generated from_json/to_json free functions that don't reference all their parameters:

inline void from_json(const json & j, Out& x) {
}

inline void to_json(json & j, const Out & x) {
    j = json::object();
}

from_json uses neither j nor x; to_json doesn't use x. Compiling with -Wextra -Wunused-parameter -Werror (a common strict build configuration) turns these into hard errors, breaking consumer builds.

Root cause

CPlusPlusRenderer.emitClassFunctions (in packages/quicktype-core/src/language/CPlusPlus/CPlusPlusRenderer.ts) always names both parameters when emitting from_json/to_json, but for a class with zero properties nothing in the body ever touches them.

Fix

In emitClassFunctions, when a class has no properties, emit (void)j; (void)x; at the top of from_json's body and (void)x; after j = json::object(); in to_json's body — the standard idiom for silencing unused-parameter warnings while keeping the parameter names (needed to match the function signature) intact.

Test coverage

The existing schema fixture test/inputs/schema/constructor.schema already generates a nested empty class (additionalProperties: false, no properties) via a oneOf union member. The C++ fixture's compile command in test/languages.ts now passes -Werror=unused-parameter, so this fixture fails to compile on unfixed code (reproducing the reported warnings) and compiles cleanly after the fix.

Verification

• Reproduced the bug locally: node dist/index.js --lang cpp --src-lang schema empty.schema.json -o out.hpp, then compiled the output with g++ -std=c++17 -Wextra -Wunused-parameter -Werror — reproduced all three reported errors (unused j and x in from_json, unused x in to_json) before the fix; no unused-parameter errors after.
• npm run build passes.
• Ran the full schema-cplusplus fixture suite locally (QUICKTEST=true FIXTURE=schema-cplusplus script/test) to confirm no regressions across the many C++ renderer option combinations.
• CI will additionally validate the broader JSON/JSON Schema fixture matrix.

Fixes #2362

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