fix: clear scan destination on NULL for LowCardinality(Nullable(T))

[polyglotAI-bot]

Description

Fixes #1932.

LowCardinality(Nullable(T)) columns track NULLs through key index 0, and during parse the wrapper disables its inner Nullable (nullable.enable = false). That means the inner Nullable.ScanRow never runs its NULL branch, so LowCardinality.ScanRow is solely responsible for resetting the scan destination on a NULL element. It did return nil without touching dest, so a *T destination reused across rows kept a stale, non-nil value after scanning a NULL. The bug is invisible when each scan targets a fresh destination — which is why it slipped past existing coverage (e.g. TestIssue751, whose destinations are re-declared per row).

The fix routes the NULL case through the same clearing logic Nullable.ScanRow already uses: a new package-level helper scanNullInto resets the **T pointer destinations and calls sql.Scanner.Scan(nil). LowCardinality.ScanRow now calls it on the NULL branch, and Nullable.ScanRow is refactored to call it too (behavior-preserving), so the two paths can no longer drift.

Changes

• lib/column/nullable.go: extract the NULL-clearing switch from Nullable.ScanRow into a shared scanNullInto(dest any) error helper (no behavior change).
• lib/column/lowcardinality.go: ScanRow now calls scanNullInto(dest) on a NULL element instead of returning without clearing dest.
• tests/issues/issue_1932_test.go: regression test.

Test

TestIssue1932_LowCardinalityNullableScanClearsDest inserts 'hi' then NULL into a LowCardinality(Nullable(String)) column, selects them ordered (ORDER BY c NULLS LAST), and scans both rows through the same reused *string. It asserts the non-null row populates the pointer ("hi") and the NULL row resets it to nil. It fails on main (Expected nil, but got: (*string)(0x...)) and passes with this change.

String is used because it is the only base type creatable under LowCardinality(Nullable(...)) on a default server — numeric / Date / DateTime base types require allow_suspicious_low_cardinality_types, which the suite avoids for ClickHouse Cloud compatibility (see #1206). Because LowCardinality.ScanRow now delegates to the very same scanNullInto helper as Nullable.ScanRow, the other clear-list arms (**int64, **float64, **time.Time, …) remain covered by the existing Nullable tests.

Verified locally against ClickHouse 26.x: the new test, the existing LowCardinality/Nullable tests (tests/, tests/std/), TestIssue751, and the lib/column unit tests all pass with no edits to existing tests.

Pre-PR validation gate

• Deterministic repro (fails on main, passes on branch)
• Root cause documented above
• Fix targets the root cause (clears dest at the responsible site)
• Regression test fails without the fix, passes with it
• No existing tests broken or weakened
• Convention compliance per AGENTS.md / .claude/CLAUDE.md (tests/issues/issue_<n>_test.go, t.Cleanup closes the conn + drops the table, version-gated)
• No public API change (scanNullInto is unexported)
• CHANGELOG.md intentionally not edited — CONTRIBUTING.md states it is generated automatically at release time