Add custom type handling for nullable fields in JSON serialization

[Zetonem]

Summary

When a glz::object field uses glz::custom<setter, getter> and the getter returns std::optional that is std::nullopt, Glaze currently serializes the field as "field": null instead of omitting the field (under skip_null_members=true). This PR adds a regression test and fixes the JSON writer so the field is skipped when the custom getter produces a null/empty optional.

Changes

1. Fix skip logic for JSON serialization

• Updated the Opts.skip_null_members handling for fields whose value type is glz::custom_t.
• For glz::custom_t, the serializer now invokes the custom getter (supports member-function/member-object pointer and invocable forms), detects whether the getter result is “null” for nullable return types (notably std::optional / optional-like via existing nullable_* concepts) and skips writing the field when the getter result is empty (std::nullopt -> omitted field)

2. Ensure glz::custom_t is treated as a candidate for skipping

Expanded the maybe_skipped condition so that when skip_null_members=true, glz::custom_t fields are considered “potentially skippable”, allowing the runtime logic in json/write.hpp to run.

Issue

#2386

[stephenberry]

@Zetonem, thanks for submitting this. I see a few completeness issues. I'll either contribute to this PR, or open a similar one and close this. But, this is helpful to see an initial solution, thanks!

[stephenberry]

I just merged in a similar fix with #2390, so I'm closing this. But, thanks for contributing and feel free to keep posting issues and PRs!

[Zetonem]

I just merged in a similar fix with #2390, so I'm closing this. But, thanks for contributing and feel free to keep posting issues and PRs!

Thank you!