Fix nested left join null mapping

[nhatlong90]

Fixes #1603.

This updates nested result mapping so a nullable joined object is only nullified when all selected fields from that joined table are null. Previously, if the first nested field was null, the mapper could nullify the whole object even when later fields from the same table had values.

Tests:

• corepack pnpm --dir drizzle-orm exec vitest run tests/utils.test.ts