buffer: special case empty string writes

Prior to 119354f we specifically handled passing a zero length string
to write on a buffer, restore that functionality.

src/node_buffer.cc

@@ -345,7 +345,15 @@ Handle<Value> Buffer::StringWrite(const Arguments& args) {
 
   Local<String> str = args[0].As<String>();
 
-  if (encoding == HEX && str->Length() % 2 != 0)
+  int length = str->Length();
+
+  if (length == 0) {
+    constructor_template->GetFunction()->Set(chars_written_sym,
+                                             Integer::New(0));
+    return scope.Close(Integer::New(0));
+  }
+
+  if (encoding == HEX && length % 2 != 0)
     return ThrowTypeError("Invalid hex string");
 
   size_t offset = args[1]->Int32Value();

test/simple/test-buffer.js

@@ -998,3 +998,10 @@ assert.equal(Buffer.byteLength('aaaa==', 'base64'), 3);
 assert.throws(function() {
   Buffer('', 'buffer');
 }, TypeError);
+
+assert.doesNotThrow(function () {
+  var slow = new SlowBuffer(1);
+  assert(slow.write('', Buffer.poolSize * 10) === 0);
+  var fast = new Buffer(1);
+  assert(fast.write('', Buffer.poolSize * 10) === 0);
+});