tdf#96246: Make pRenderingArguments nullable

Type gchar* is not nullable by convention.

See:
https://wiki.gnome.org/Projects/GObjectIntrospection/Annotations#Nullable_parameters

Change-Id: Ibfee816a3ef2d29c7376071fb61eda7bf0538efb
Reviewed-on: https://gerrit.libreoffice.org/20425Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
(cherry picked from commit 9e970338)

include/LibreOfficeKit/LibreOfficeKitGtk.h

@@ -65,7 +65,7 @@ GtkWidget*                     lok_doc_view_new_from_widget        (LOKDocView*
  * lok_doc_view_open_document:
  * @pDocView: The #LOKDocView instance
  * @pPath: (transfer full): The path of the document that #LOKDocView widget should try to open
- * @pRenderingArguments: lok::Document::initializeForRendering() arguments.
+ * @pRenderingArguments: (nullable): lok::Document::initializeForRendering() arguments.
  * @cancellable:
  * @callback:
  * @userdata: