fix: guard optional onExit cleanup handler

[pupuking723]

Fixes #84.

signal-exit can return undefined from onExit() in some versions/environments. write-file-atomic currently assumes the return value is always a remover function, so the async and sync paths can throw while leaving the finally cleanup section even when the write itself succeeded.

This adds a no-op fallback for the optional remover and covers both async and sync writes with a regression test that mocks onExit() to return undefined.

Verification:

• npx tap --no-coverage test/basic.js
• npm run eslint -- lib/index.js test/basic.js
• git diff --check
• npm test passes tests, coverage, and eslint locally, then fails only in template-oss-check because the generated repository template wants unrelated .github branch/template updates. I did not include those unrelated tooling changes in this bugfix PR.