Workaround for root spans with alien `FOLLOWS_FROM` references

[swythan]

Which problem is this PR solving?

• Workaround/solution for #2427

Description of the changes

These were the changes I had to make locally (in the Chrome debug tools) to prevent the page from crashing when the root span had a FOLLOWS_FROM reference from a different trace.

How was this change tested?

Only by making these changes directly in the minified sources in Chrome/Edge's "Local Overrides". It stopped things failing, but I'm not sure whether the actual critical path algorithm still works afterwards.

Checklist

• [x] I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• [ ] I have signed all commits
• [ ] I have added unit tests for the new functionality
• [ ] I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: yarn lint and yarn test