hugolib: Allow nil to be unwrapped as *Page

Previously, calls to *Page.Eq(nil) would always return false because the
unwrapPage func didn't support the nil case.  Add support for unwrapping
nil to a *Page.

Fixes #5043

hugolib/site_sections.go

@@ -152,6 +152,8 @@ func unwrapPage(in interface{}) (*Page, error) {
 		return v.Page, nil
 	case *PageWithoutContent:
 		return v.Page, nil
+	case nil:
+		return nil, nil
 	default:
 		return nil, fmt.Errorf("%T not supported", in)
 	}