Clear'ing empty test tab

Currently, clearing the test tab works via

auto success = mUi->tblFailedChecks->model()->removeRows(0, mUi->tblFailedChecks->model()->rowCount());

But with rowCount = 0 this leads to the call beginRemoveRows(parent, row, row + count - 1) = beginRemoveRows(parent, 0, - 1), which is invalid.