Resolve "Test case without tasks shows "0 of 0 tasks completed""

What does this MR do and why?

Previously, test cases always had an information about tasks on header, even if test case had no tasks. This MR removes this message and shows it only when tasks are added to test case. Additionally, I've fixed a bug where after editing test case description and adding a task list count didn't update - we needed to return the information about tasks count with the updating mutation response

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. With an Ultimate or Gold license, create a demo project
2. Create a test case.
3. Check that there is no message about tasks
4. Edit the description to add the following text:

   - [ ] task

5. There should be 0 of 1 task completed message in the header

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #356563 (closed)