Return if null

[mgifford]

I think that we improve performance a bit here.

This PR adds...

• [x] I've kept this PR as small as possible (~500 lines) by splitting it into PRs with manageable chunks of code
• [ ] I've requested reviews from 1 reviewer
• [ ] I've tested existing features (website scan, sitemap, custom flow) in both node index and cli
• [ ] I've synced this fork with GovTechSG repo
• [ ] I've added/updated unit tests
• [ ] I've added/updated any necessary dependencies in package[-lock].json npm audit, portable installation on GitHub Actions