Fix nested left join null detection

[lukitadproxd-netizen]

Closes #1603

Summary

• Fixes nested select result mapping for left joins when the first selected column from a joined table is null but later columns from the same table are not null.
• Keeps nullable joined objects only when at least one selected column from that table has a non-null value.
• Adds regression coverage for both the partial-null and all-null joined object cases.

Test plan

• corepack pnpm install --frozen-lockfile --ignore-scripts
• corepack pnpm --dir drizzle-orm exec vitest run tests/utils.test.ts
• corepack pnpm exec dprint check drizzle-orm/src/utils.ts drizzle-orm/tests/utils.test.ts

Note: local full install with lifecycle scripts is blocked on this Windows/Node 24 environment by native optional dependency builds (better-sqlite3/cpu-features). The focused test above is pure and passes locally.

/claim #1603