fix: handle nullable validity layers without def levels

[Xuanwo]

This fixes the reader panic in #6185 when a page keeps nullable rep/def layer metadata but does not materialize any definition levels. The decoder now treats that page-local state as all-valid and includes a regression test that reproduces the mixed-page case before the fix.

Closes #6185.

[github-actions]

Review

Clean, well-scoped fix for the panic in #6185. The approach of treating def_levels: None as all-valid is correct — if no definition buffer was materialized, there are no nulls to decode.

One minor observation:

In skip_validity and the early-return path of unravel_validity, the fix advances current_def_cmp += meaning.num_def_levels(). When def_levels is None, this counter is effectively dead (all subsequent layers will also hit the def_levels.is_none() bail-out), so it's harmless — but it's also good practice to keep the state consistent in case the invariants change later. Looks fine as-is.

The regression test correctly covers the mixed-page scenario (one page with def levels, one without). LGTM.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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