org-element-fixed-width-parser: Fix when :value is empty

* lisp/org-element.el (org-element-fixed-width-interpreter): Interpret
fixed-width element with :value "" as ":\n", not as empty string.
* testing/lisp/test-org-element.el (test-org-element/fixed-width-interpreter):
Fixed the test.

The previous test, albeit intentionally asserting "" result, breaks
reversibility of parse-interpret sequence.

lisp/org-element.el

@@ -2382,7 +2382,9 @@ Assume point is at the beginning of the fixed-width area."
 (defun org-element-fixed-width-interpreter (fixed-width _)
   "Interpret FIXED-WIDTH element as Org syntax."
   (let ((value (org-element-property :value fixed-width)))
-    (and value (replace-regexp-in-string "^" ": " value))))
+    (and value
+         (if (string-empty-p value) ":\n"
+           (replace-regexp-in-string "^" ": " value)))))
 
 
 ;;;; Horizontal Rule

testing/lisp/test-org-element.el

@@ -3099,7 +3099,7 @@ CLOCK: [2012-01-01 sun. 00:01]--[2012-01-01 sun. 00:02] =>  0:01"))))
   (should
    (equal (org-element-fixed-width-interpreter
 	   '(fixed-width (:value "")) nil)
-	  ""))
+	  ":\n"))
   ;; Handle nil value.
   (should-not
    (org-element-fixed-width-interpreter