Resolve "Assign quick action does not load the list of users"

What does this MR do?

Fix the /assign @ dropdown on New issue page.

Provide default empty array to prevent assignees.includes from throwing

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry

• Code review guidelines

• Merge request performance guidelines

• Style guides

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

• Tested in all supported browsers

Closes #210005 (closed)