ox-md: Prevent error with empty items

* lisp/ox-md.el (org-md-item): Do not return an error when exporting
  an empty item.

lisp/ox-md.el

@@ -244,7 +244,8 @@ a communication channel."
 	      (off "[ ] "))
 	    (let ((tag (org-element-property :tag item)))
 	      (and tag (format "**%s:** "(org-export-data tag info))))
-	    (org-trim (replace-regexp-in-string "^" "    " contents)))))
+	    (and contents
+		 (org-trim (replace-regexp-in-string "^" "    " contents))))))
 
 
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