Python stubs for QItemSelection.__init(self) returning wrong type

The 16429c860888578b5bda7fd75437dd1f6a79b331 correctly fixes the the empty constructor not being seen by Shiboken by declaring it explicitly in Type System Reference:

<!-- For some reason, the empty selection is not seen. Maybe related to the new [default] tag in Qt6? -->
  <declare-function signature="QItemSelection()" return-type="QItemSelection" />

But signature generation system now passes the return type of such function as being QItemSelection, which is not correct for constructor calls from the Python POV. The Type System Reference should declare it while modifying the pyi-type type to None.