Fix(MVC) Assignee(s) avatar dissapear when opening issue sidebar in board

What does this MR do?

This fix is a MVC for #197894 (closed) as its been reported by customers.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Test case added for issue_card_spec to simulate when the assignee's list is updated and assignee's now contains a avatar_url property and not a avatar property.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers