[regexp] fix zero-length matches for RegExp.prototype.@@split.

test/mjsunit/harmony/unicode-regexp-zero-length.js

Index: test/mjsunit/harmony/unicode-regexp-zero-length.js
diff --git a/test/mjsunit/harmony/unicode-regexp-zero-length.js b/test/mjsunit/harmony/unicode-regexp-zero-length.js
index 21b08eb15ae660661ccdce45f2043203971d51a4..bbc17dc2d52fe37654e3f667c937141e8298e592 100644
--- a/test/mjsunit/harmony/unicode-regexp-zero-length.js
+++ b/test/mjsunit/harmony/unicode-regexp-zero-length.js
@@ -53,3 +53,6 @@ assertEquals(["","",""], (L + T + L + T).match(u));
 var expected = [];
 for (var i = 0; i <= 1000; i++) expected.push("");
 assertEquals(expected, (L + T).repeat(1000).match(u));
+
+// Also test RegExp.prototype.@@split.
+assertEquals(["\u{12345}"], "\u{12345}".split(/(?:)/u));