Manipulating NULL iterator should be concidered as a defined behaviour as it

Manipulating NULL iterator should be concidered as a defined behaviour as it
give the possibility to write small code like :

it = eina_hash_iterator_tuple_new(hash);
eina_iterator_foreach(it, do_something_cb, NULL);
eina_iterator_free(it);

If hash is empty, but valid it will return a NULL iterator for this
example.

git-svn-id: svn+ssh://svn.enlightenment.org/var/svn/e/trunk/eina@38104 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33