Fix the work items export csv modal items count always showing 0

What does this MR do and why?

Fixes the work items export csv modal items count, when planning view feature flag is enabled

currently it always shows "0 work items selected" on work items page.

References

#583177

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Checkout this branch and run gdk locally
2. Enable planning view feature flag and go to work items page
3. Open work items csv export modal by clicking on three dot menu > export work items csv
4. Notice the count, it no longer says "0 work items selected" but shows actual count.
5. Compare with master branch, where it always shows "0 work items selected"