Nullable value write skipping

[stephenberry]

Fix skip_null_members for glaze_value_t wrapping nullable types

Fixes #2397

When a struct field uses glaze::value to expose a nullable inner type (e.g. std::optional, std::unique_ptr), the skip_null_members option was not skipping the field during writing. The null check was only performed on the outer wrapper type, which is not itself nullable.

Changes

include/glaze/core/reflect.hpp

• Added glaze_value_is_nullable<V>() — compile-time check for whether a glaze_value_t wraps a nullable inner type.
• Added is_glaze_value_field_null<T, I>(value, t) — runtime null check that unwraps the value type and tests the inner value.
• Updated maybe_skipped to recognize nullable value types so the skip code path is entered.

include/glaze/json/write.hpp, cbor/write.hpp, beve/write.hpp

• Added an else if branch in each object-member write loop that calls the new helpers to skip null value-type fields. CBOR and BEVE each have two sites (member-count pass and write pass).

Tests

• std::optional<int> inner type — null skipped, non-null written
• std::unique_ptr<int> inner type — exercises the !bool() null-check path
• Multiple nullable value-type fields with mixed null/non-null
• skip_null_members = false — null explicitly written as null
• JSON and BEVE round-trip (write then read back)