Fix behavour of node index for userData.txt not created and run in background

[younglim]

This PR adds...

• Fixes behaviour of node index error due to userData not created on first run, but works on subsequent run
• Fixes run in background in node index

• [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
• [x] 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