Allow to not have empty Ptyp_poly node in the Parsetree (the type-checker insert them automatically).

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13491 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Doc.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13492 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02