bug#24252: 25.1; json.el doesn't distinguish null and empty object

[Yoichi Nakayama]

The change in json-encode-key broke tests in json-tests.el. I'll investigate it.

Ran 34 tests, 28 results as expected, 6 unexpected (2016-08-21 22:30:23+0900)

6 unexpected results:
   FAILED  test-json-encode-alist-with-sort-predicate
   FAILED  test-json-encode-hash-table
   FAILED  test-json-encode-key
   FAILED  test-json-encode-list
   FAILED  test-json-encode-plist
   FAILED  test-json-encode-plist-with-sort-predicate

-- 
Yoichi NAKAYAMA