Allow returning empty list from post-processing block

* lisp/ob-ref.el (org-babel-ref-resolve): When the result an empty
list, then treat it as a list, not as the symbol nil.
* testing/lisp/test-ob.el (test-ob/post-header-arguments): Add new
test.
* etc/ORG-NEWS (Post-processing code blocks can return an empty list):
Document change in behavior.

etc/ORG-NEWS

@@ -425,6 +425,12 @@ The new variable name is =org-plantuml-args=.  It now applies to both
 jar PlantUML file and executable.
 
 ** Miscellaneous
+*** Post-processing code blocks can return an empty list
+
+When the result of a regular code block is nil, then that was already
+treated as an empty list.  Now that is also the case for code blocks
+that post-process the result of another block.
+
 *** Styles are customizable in ~biblatex~ citation processor
 
 It is now possible to add new styles or modify old ones in ~biblatex~

lisp/ob-ref.el

@@ -199,7 +199,7 @@ Emacs Lisp representation of the value of the variable."
 				(org-babel-execute-src-block nil info params))))
 		     (error "Reference `%s' not found in this buffer" ref))))
 	     (cond
-	      ((symbolp result) (format "%S" result))
+	      ((and result (symbolp result)) (format "%S" result))
 	      ((and index (listp result))
 	       (org-babel-ref-index-list index result))
 	      (t result)))))))))

testing/lisp/test-ob.el

@@ -136,6 +136,30 @@ should still return the link."
     (let ((info (org-babel-get-src-block-info)))
       (should (string= "no" (cdr (assq :tangle (nth 2 info))))))))
 
+(ert-deftest test-ob/post-header-arguments ()
+  "When the result of a post-processing source block is an empty
+list, then it should be treated as such; not as the symbol nil."
+  (should
+   (let ((default-directory temporary-file-directory))
+     (org-test-with-temp-text
+         "
+#+name: addheader
+#+header: :var rows=\"\"
+#+begin_src elisp :hlines yes
+  '()
+#+end_src
+#+header: :post addheader(*this*)
+#+<point>begin_src emacs-lisp :results table
+#+end_src
+#+RESULTS:
+: nil"
+       (org-babel-execute-src-block)
+       (goto-char (1- (point-max)))
+       (equal (buffer-substring-no-properties
+               (line-beginning-position)
+               (line-end-position))
+              "#+RESULTS:")))))
+
 (ert-deftest test-ob/elisp-in-header-arguments ()
   "Test execution of elisp forms in header arguments."
   (org-test-with-temp-text-in-file "