Fix QAbstractItemModelTester false positive
When inserting rows to a branch with no columns the tester should not complain about indexes being invalid
Pick-to: 6.1 6.0 5.15 Change-Id: I466f4e5140b10f6dcf65a71f109c2d3be7336507 Reviewed-by: David Faure david.faure@kdab.com (cherry picked from commit fcea8e7a)