Exit mergeChildrenCriticalPath early for an empty critical path (#1974)

## Which problem is this PR solving?
- Part of #1943 

## Description of the changes
- mergeChildrenCriticalPath shortcuts for an empty critical path

## How was this change tested?
- Existing tests

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

---------

Signed-off-by: Maksim Gaponov <gaponovmaxev@gmail.com>
Co-authored-by: Yuri Shkuro <yurishkuro@users.noreply.github.com>

packages/jaeger-ui/src/components/TracePage/TraceTimelineViewer/VirtualizedTraceView.tsx

@@ -163,6 +163,9 @@ function mergeChildrenCriticalPath(
   spanID: string,
   criticalPath: criticalPathSection[]
 ): criticalPathSection[] {
+  if (!criticalPath) {
+    return [];
+  }
   // Define an array to store the IDs of the span and its descendants (if the span is collapsed)
   const allRequiredSpanIds = [spanID];