This patch mostly introduces some test improvements to check the
calls of different methods on an empty default-constructed
container.
Apart from that some other tests are added to extend test
coverage.
Task-number: QTBUG-91736
Pick-to: 6.2 6.1
Change-Id: If2bc96158462292bbdf8504942141af94568c729
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>