LPS-49911 REGRESSION: Pages of type "Link to another page in the current site" does not work as expected

Regression caused by LPS-46493.

For pages of type "Link to Page" the plid value is zero, but after LPS-46493 changes, plid is always setted with "layout.getPlid()".

Because this change, for that kind of pages, "processLayout" is executed instead "forwardLayout"