* lisp/babel/ob-exp.el (org-babel-exp-results): now checking to see if
return value of `org-babel-ref-literal' is equal to the null
indicator flag -- meaning it's now possible to accept a value of
null
* lisp/babel/ob-ref.el (org-babel-ref-parse): now checking to see if
return value of `org-babel-ref-literal' is equal to the null
indicator flag -- meaning it's now possible to accept a value of
null
(org-babel-ref-literal): now returning a null indicator flag when
the value is *not* a literal value, meaning it is possible to pass
in a literal value of null