Fix that the placeholdertext of QCombobox is not drawn
If the current index is invalid,a placeholdertext will be displayed.
Fixes: QTBUG-90595 Change-Id: Id4c6b9c959242b96ee0944b8fc3131a9a2fdcccc Reviewed-by: Christian Ehrlicher ch.ehrlicher@gmx.de (cherry picked from commit 6da6b6da) Reviewed-by: Qt Cherry-pick Bot cherrypick_bot@qt-project.org (cherry picked from commit 191e1bbf)