type hints: add explicit optional to setItemWidget

Change-Id: I9d14b23d9fe2e084ffdacd66560c84c8c0d54764
Fixes: PYSIDE-3042
Pick-to: 6.8
Reviewed-by: Ece Cinucen <[email protected]>
Reviewed-by: Friedemann Kleint <[email protected]>

Author: cmaureir

sources/pyside6/PySide6/QtWidgets/typesystem_widgets_common.xml

@@ -1957,7 +1957,7 @@
       </modify-argument>
     </modify-function>
     <modify-function signature="setItemWidget(QListWidgetItem*,QWidget*)" allow-thread="yes">
-      <modify-argument index="2">
+      <modify-argument index="2" pyi-type="Optional[PySide6.QtWidgets.QWidget]">
         <parent index="1" action="add"/>
       </modify-argument>
     </modify-function>