[string] Fix null handling in MaybeCallFunctionAtSymbol

test/mjsunit/es6/string-split.js

Index: test/mjsunit/es6/string-split.js
diff --git a/test/mjsunit/es6/string-split.js b/test/mjsunit/es6/string-split.js
index 8ca655cad9a04246c4641fff24dfaeade0c074c0..c21f9d3d9479628a5b067d1d4e960370f2d72855 100644
--- a/test/mjsunit/es6/string-split.js
+++ b/test/mjsunit/es6/string-split.js
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-var pattern = {};
+var pattern = {toString: () => ""};
 var limit = { value: 3 };
 pattern[Symbol.split] = function(string, limit) {
   return string.length * limit.value;
@@ -15,5 +15,8 @@ assertEquals(15, "abcde".split(pattern, limit));
 // Non-callable override.
 pattern[Symbol.split] = "dumdidum";
 assertThrows(() => "abcde".split(pattern, limit), TypeError);
+// Null override.
+pattern[Symbol.split] = null;
+assertEquals(["a", "b", "c", "d", "e"], "abcde".split(pattern));
 
 assertEquals("[Symbol.split]", RegExp.prototype[Symbol.split].name);