Fix nested partial select nullification on left join

[cjnegao11-cmyk]

Summary

• prevent nested partial-select objects from being nullified when a later column from the same joined table is non-null
• add unit coverage for mixed null/non-null joined fields and all-null joined fields
• add a PostgreSQL integration regression test for the reported left join scenario

Validation

• pnpm exec vitest run tests/utils.test.ts → 2 passed
• pnpm exec tsc --noEmit --pretty false -p tests/tsconfig.json → passed
• package test suite executed 544 tests successfully; 2 SQLite casing suites could not start locally because the native better-sqlite3 binding was unavailable on Windows

Closes #1603

[cjnegao11-cmyk]

This PR addresses #1603 by updating the nested result nullification logic and adding regression coverage for the reported left join case.