Avoid erroring on nullable value types

[stephenberry]

Fix error_on_missing_keys for value types wrapping nullable types

Fixes #2389

When a struct uses glaze::value to serialize as one of its members (a "value type"), and that member is nullable (e.g. std::optional), error_on_missing_keys incorrectly treated the field as required. This is because required_fields() checked nullability on the outer struct rather than the underlying type it serializes as.

Changes

include/glaze/core/reflect.hpp — Added a glaze_value_t branch to required_fields() that unwraps the value type via remove_meta_wrapper_t and checks nullability on the inner type. Applied in both the meta_has_requires_key path and the default path.

tests/json_test/json_test.cpp — Added a test verifying that a missing value-type field backed by std::optional does not produce a missing_key error, while non-nullable required fields still do.