org-element: Fix interpretation of empty fixed-width elements

* lisp/org-element.el (org-element-fixed-width-interpreter): Fix
  interpretation of fixed-width elements with a nil or empty string
  value.
* testing/lisp/test-org-element.el: Add tests.

lisp/org-element.el

@@ -1834,8 +1834,11 @@ Assume point is at the beginning of the fixed-width area."
 (defun org-element-fixed-width-interpreter (fixed-width contents)
   "Interpret FIXED-WIDTH element as Org syntax.
 CONTENTS is nil."
-  (replace-regexp-in-string
-   "^" ": " (substring (org-element-property :value fixed-width) 0 -1)))
+  (let ((value (org-element-property :value fixed-width)))
+    (and value
+	 (replace-regexp-in-string
+	  "^" ": "
+	  (if (string-match "\n\\'" value) (substring value 0 -1) value)))))
 
 
 ;;;; Horizontal Rule

testing/lisp/test-org-element.el

@@ -2185,7 +2185,25 @@ CLOCK: [2012-01-01 sun. 00:01]--[2012-01-01 sun. 00:02] =>  0:01"))
   (should (equal (org-test-parse-and-interpret ": Test") ": Test\n"))
   ;; Preserve indentation.
   (should (equal (org-test-parse-and-interpret ":  2 blanks\n: 1 blank")
-		 ":  2 blanks\n: 1 blank\n")))
+		 ":  2 blanks\n: 1 blank\n"))
+  ;; Remove last newline character
+  (should
+   (equal (org-element-fixed-width-interpreter
+	   '(fixed-width (:value "Test\n")) nil)
+	  ": Test"))
+  (should
+   (equal (org-element-fixed-width-interpreter
+	   '(fixed-width (:value "Test")) nil)
+	  ": Test"))
+  ;; Handle empty string.
+  (should
+   (equal (org-element-fixed-width-interpreter
+	   '(fixed-width (:value "")) nil)
+	  ""))
+  ;; Handle nil value.
+  (should-not
+   (org-element-fixed-width-interpreter
+    '(fixed-width (:value nil)) nil)))
 
 (ert-deftest test-org-element/horizontal-rule-interpreter ()
   "Test horizontal rule interpreter."