[BUG] org-babel-script-escape mishandled empty python list

[Jonas Bernoulli]

`org-babel-script-escape' doesn't handle an empty python list
correctly; ['a'] => ("a") but [] => [].

#+name: addheader
#+header: :var rows=""
#+begin_src python
  return []
#+end_src

#+header: :post addheader(*this*)
#+begin_src emacs-lisp :results table
  nil
#+end_src
#+RESULTS:
: []

An empty python list is turned into an empty lisp vector instead of
an empty lisp list.  At least for python, (> (length str) 2) should
probably be changed to use >=.