ob-core: Do not return results on writing to file

* ob-core (org-babel-result-cond): Add "file" as a param that returns
  a scalar result.

When writing the result to file we do not need to first convert the
data to a table since it is just converted back when
org-babel-execute-src-block serializes the result to file.

lisp/ob-core.el

@@ -2979,6 +2979,7 @@ Emacs shutdown."))
 		 (member "html" ,params)
 		 (member "code" ,params)
 		 (member "pp" ,params)
+		 (member "file" ,params)
 		 (and (or (member "output" ,params)
 			  (member "raw"    ,params)
 			  (member "org"    ,params)