add empty replace test

test/built-ins/RegExp/named-groups/string-replace-unclosed.js

@@ -21,4 +21,6 @@ for (let flags of ["", "u", "g", "gu"]) {
   let re = new RegExp(source, flags);
   assert.throws(SyntaxError, () => "abcd".replace(re, "$<snd"),
                 "unclosed named group in replacement should throw a SyntaxError");
+  assert.throws(SyntaxError, () => "abcd".replace(re, "$<>"),
+                "empty named group in replacement should throw a SyntaxError");
 }