ob-ocaml.el: Make sure a value was extracted before printing

* lisp/ob-ocaml.el (org-babel-execute:ocaml): Do not try to print
a value if either the type or the value could not be extracted from
the toplevel.
org-element: Slightly change object cache

* lisp/org-element.el (org-element--cache-objects):
(org-element-context): Simplify structure of object cache.

This change avoids relying on buffer positions and artificial keys.
org-element: Small fix

* lisp/org-element.el (org-element--cache-process-request): Small fix.

This fixes 7fca554f434c1659adb8a931eec590784a217e86.