test: cover export dump null serialization

[thedalbee]

/claim #71

Summary

Adds a focused, non-overlapping coverage slice for SQL export dumps in src/export/dump.ts.

Covered behavior:

• NULL cell values now serialize as SQL NULL instead of producing invalid empty INSERT values like INSERT INTO users VALUES (1, );.
• A table whose schema lookup returns no rows still emits its row data instead of failing or stopping the dump.
• Existing string escaping behavior remains covered.

This is scoped to export dump generation and intentionally avoids the active CSV/import/RLS/cache/allowlist/operation/worker/handler/DO/LiteREST/plugin coverage PR areas.

Verification

• pnpm exec vitest run src/export/dump.test.ts - passed, 7 tests.
• pnpm exec vitest run src/export/dump.test.ts --coverage.enabled true --coverage.include src/export/dump.ts --coverage.reporter text --coverage.reportsDirectory coverage-export-dump --coverage.thresholds.lines=0 --coverage.thresholds.branches=0 --coverage.thresholds.functions=0 --coverage.thresholds.statements=0 - passed; src/export/dump.ts at 100% statements, branches, functions, and lines.
• pnpm exec prettier --check src/export/dump.ts src/export/dump.test.ts - passed.
• git diff --check - passed.
• pnpm exec vitest run - export dump suite passed; current repo baseline still has 4 unrelated src/rls/index.test.ts failures.
• pnpm exec tsc --noEmit - current repo baseline has unrelated type errors in plugins/cdc, src/cache/index.test.ts, src/do.test.ts, and src/operation.ts.

Demo video

• Short terminal result demo: https://raw.githubusercontent.com/thedalbee/starbasedb/demo-export-dump-coverage-71/starbasedb-export-dump-coverage-demo.mp4