org-element: Fix footnote definition parsing

* lisp/org-element.el (org-element-footnote-definition-parser): Fix
  value for :contents-begin when first line of footnote definition is
  empty besides the label.
* testing/lisp/test-org-element.el (test-org-element/footnote-definition-parser):
  Add test.

lisp/org-element.el

@@ -683,9 +683,12 @@ Assume point is at the beginning of the footnote definition."
 				    "^\\([ \t]*\n\\)\\{2,\\}") limit 'move))
 			 (match-beginning 0)
 		       (point))))
-	   (contents-begin (progn (search-forward "]")
-				  (skip-chars-forward " \r\t\n" ending)
-				  (and (/= (point) ending) (point))))
+	   (contents-begin (progn
+			     (search-forward "]")
+			     (skip-chars-forward " \r\t\n" ending)
+			     (cond ((= (point) ending) nil)
+				   ((= (line-beginning-position) begin) (point))
+				   (t (line-beginning-position)))))
 	   (contents-end (and contents-begin ending))
 	   (end (progn (goto-char ending)
 		       (skip-chars-forward " \r\t\n" limit)

testing/lisp/test-org-element.el

@@ -760,21 +760,26 @@ Some other text
   "Test `footnote-definition' parser."
   (should
    (org-test-with-temp-text "[fn:1] Definition"
-     (org-element-map
-      (org-element-parse-buffer) 'footnote-definition 'identity nil t)))
+     (org-element-map (org-element-parse-buffer) 'footnote-definition
+       'identity nil t)))
   ;; Footnote with more contents
   (should
    (= 29
       (org-element-property
        :end
        (org-test-with-temp-text "[fn:1] Definition\n\n| a | b |"
-	 (org-element-map
-	  (org-element-parse-buffer)
-	  'footnote-definition 'identity nil t)))))
+	 (org-element-map (org-element-parse-buffer) 'footnote-definition
+	   'identity nil t)))))
   ;; Footnote starting with special syntax.
   (should-not
    (org-test-with-temp-text "[fn:1] - no item"
-     (org-element-map (org-element-parse-buffer) 'item 'identity))))
+     (org-element-map (org-element-parse-buffer) 'item 'identity)))
+  ;; Correctly handle footnote starting with an empty line.
+  (should
+   (= 9
+      (org-test-with-temp-text "[fn:1]\n\n  Body"
+	(org-element-property :contents-begin
+			      (org-element-at-point))))))
 
 
 ;;;; Footnotes Reference.