Extend tests to explicitly check the behavior of empty
default-constructed container.
Also add some missing tests to increase the code coverage.
Task-number: QTBUG-91736
Pick-to: 6.2 6.1
Change-Id: Ib3c3f5bcb967cd1031cb24ffceaa77a146212ffd
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>