fix(cpp): compile optional any-typed properties with --hide-null-optional

[schani]

The bug

When generating C++ (nlohmann, either boost or --no-boost) with
--hide-null-optional, an optional property with no declared type
("any", e.g. an empty {} schema) produces a to_json guard that does
not compile:

if (x.get_alpha()) {
    j["alpha"] = x.get_alpha();
}

get_alpha() returns const nlohmann::json &. nlohmann::json
deliberately has no operator bool() (nlohmann/json#951), so this is a
compile error.

Root cause

For every other type, an optional property is wrapped in
std::optional<T> / std::shared_ptr<T>, whose operator bool() makes
if (x.get_foo()) valid. But any-typed properties are represented as a
plain, unwrapped nlohmann::json member (see cppType() in
CPlusPlusRenderer.ts, which forces isOptional = false for AnyType).
The to_json emitter's --hide-null-optional guard
(packages/quicktype-core/src/language/CPlusPlus/CPlusPlusRenderer.ts)
didn't account for this and always emitted the if (x.get_<name>())
pattern for any optional property, regardless of whether the underlying
C++ type supports contextual bool conversion.

The fix

For optional properties whose type is null or any (i.e. backed by a
bare nlohmann::json value, not an optional/shared_ptr wrapper), the
--hide-null-optional guard now tests !value.is_null() instead of
relying on an implicit bool conversion. This compiles correctly and
preserves the intended "omit the key when the value is absent/null"
behavior. All other optional property types are unaffected and keep the
existing if (x.get_foo()) guard.

Test coverage

Extended the existing test/inputs/schema/optional-any.schema JSON
Schema fixture with:

• a new required bar: boolean property alongside the existing optional
  untyped foo property (updated .1.json/.2.json samples
  accordingly),
• a new .3.fail.json negative sample (bar given a non-boolean value,
  expected to fail),
• a C++ quickTestRendererOptions entry in test/languages.ts pinning
  hide-null-optional: true for this schema, so the C++ schema fixture
  actually exercises --hide-null-optional against an any-typed
  property (previously nothing in the C++ fixture suite turned that
  option on for this schema).

Verification

• Confirmed the bug on unfixed master by regenerating the reporter's
  repro schema with --lang c++ --hide-null-optional --no-boost and
  observing the invalid if (x.get_alpha()) guard in the output.
• After the fix, regenerated the same repro and confirmed the guard is
  now if (!x.get_alpha().is_null()), and other optional properties
  (e.g. an int64_t) are unaffected.
• npm run build passes.
• Ran the full C++ schema fixture suite locally (QUICKTEST=true FIXTURE=schema-cplusplus script/test, compiling with g++): all 72
  tests pass, including the new optional-any.schema case with
  hide-null-optional: true.
• Since optional-any.schema is a shared fixture input used by many
  other language schema fixtures, also ran schema-python and
  schema-typescript locally to confirm the input change doesn't
  regress other languages: both pass. CI will validate the remaining
  languages that use this fixture.

Fixes #1961.

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