JSON parser: fix empty dictionaries inside of dictionaries

I broke this with my change to stop boxing everything. It was dropping
empty dictionaries inside dictionaries, because the "}" of empty
dictionaries is a different parser action from the "}" of nonempty
dictionaries, and I'd neglected to update it. (The fact that no tests
caught this before is a little unsettling.)

hphp/runtime/ext/JSON_parser.cpp

@@ -551,6 +551,8 @@ bool JSON_parser(Variant &z, const char *p, int length, bool assoc/*<fb>*/,
           empty }
         */
       case -9:
+        attach_zval(the_json, JSON(the_kstack)[JSON(the_top)], assoc);
+
         if (!pop(the_json, MODE_KEY)) {
           return false;
         }

hphp/test/slow/json/emptydict.php

@@ -0,0 +1,11 @@
+<?php
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+var_dump(json_decode(
+           <<<EOT
+{
+   "staticBitmask": {},
+   "spaceId": 3
+}
+EOT
+           , true));

hphp/test/slow/json/emptydict.php.expect

@@ -0,0 +1,7 @@
+array(2) {
+  ["staticBitmask"]=>
+  array(0) {
+  }
+  ["spaceId"]=>
+  int(3)
+}