custom optional support

[stephenberry]

Fix glz::custom getter returning nullopt not being omitted

Fixes #2386

When a glz::object field uses glz::custom<setter, getter> and the getter returns a nullable type (e.g. std::optional<T>), the field was always serialized — even when the getter returned std::nullopt. With skip_null_members = true (the default), the expected behavior is to omit the field entirely, consistent with how plain std::optional members and nullable lambdas are handled.

The root cause is that the skip-null check in the object write path operates on field_t, which for custom fields is custom_t<...> — a wrapper that does not satisfy null_t. The nullable return type of the getter is never inspected.

Changes

include/glaze/core/reflect.hpp

• Added custom_getter_returns_nullable<V>(): compile-time trait that inspects the getter's return type across all supported forms (member function pointers, member object pointers, std::function members, concrete lambdas, generic lambdas, context-taking invocables).
• Added custom_getter_is_null(): runtime helper that invokes the getter and checks whether the result is null. Reused by all three format writers.
• Updated maybe_skipped to include custom_t fields whose getters return nullable types, so the dynamic-skip path is entered when needed.

include/glaze/json/write.hpp

• Added an else if constexpr branch for custom_t with nullable getter in the object field write loop. Invokes the getter via custom_getter_is_null() and skips the field if null.

include/glaze/cbor/write.hpp

• Same fix in both the count pass and write pass of the CBOR object serializer.

include/glaze/beve/write.hpp

• Same fix in both the count pass and write pass of the BEVE object serializer.

tests/json_test/nullable_lambda_test.cpp

• Added 3 test cases using glz::custom<setter, getter> with an std::optional<std::string> getter:
  • Getter returns std::nullopt → field omitted
  • Getter returns a value → field written
  • skip_null_members = false → field written as null