fix(query): respect null ordering in topn pruner

[sundy-li]

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Summary

• Ensure TopN pruning respects NULL ordering so DESC queries with NULLS LAST don't drop all non-null blocks
• Add a sqllogic regression that reproduces the nullable timestamp scenario seen on population_all_columns

Changes

The pruner now compares block statistics through a null-aware helper. This guarantees that when NULLS LAST is requested, blocks containing real timestamps remain eligible even if some segments only contain NULL stats. A new sqllogic test writes multiple blocks (with compaction disabled) and verifies ORDER BY ts DESC keeps the newest non-NULL rows ahead of NULL rows.

Implementation

1. Added compare_scalar_for_sorting and compare_block_stats to centralize NULL-first/last handling inside the TopN pruner
2. Added a regression unit test plus helpers for constructing NULL-only block stats
3. Extended tests/sqllogictests/suites/query/order.test with a nullable timestamp table flow matching the production query

Tests

• Unit Test (cargo test -p databend-storages-common-pruner)
• Logic Test (make sqllogic-test) — not run in this environment
• Benchmark Test
• No Test - Pair with the reviewer to explain why

Type of change

• Bug fix (non-breaking change which fixes an issue)

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🤖 CI Job Analysis

Workflow: 21333729791

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• Total Jobs: 84
• Failed Jobs: 1
• Retryable: 0
• Code Issues: 1

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