Upgrading VTU to v1: Remove deprecated `isEmpty` call

What does this MR do?

As simple as it is. @vue/test-utils 1.x deprecate .isEmpty().

This MR replaces all remaining calls with meaningful assertions or drop them if they make no sense in the context of the test

No other changes :)

See &3856 (closed) for details

Screenshots

N/A

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done