Only operate on non-empty input

src/core/operations/XMLValidator.mjs

@@ -35,6 +35,7 @@ class XMLValidator extends Operation {
      */
     run(input, args) {
 
+        if (input)
             try {
                 // Overwrite error handler since the built-in one does not raise exceptions.
                 (new DOMParser.DOMParser({errorHandler: {