Fixed invalid JSON parsing for empty attributes

https://github.com/erichexter/twitter.bootstrap.mvc/commit/0ffe23fb23b6e3a6f32455ffd767ae0d308ead28

An empty string (or null object) is invalid JSON, and needs to be
converted to a 'null' string. jQuery 1.9.0 has exposed this previously
accepted JSON. Now we need to convert it.

http://forum.jquery.com/topic/parsejson-in-1-9-0-is-different-from-earlier-versions
Currently, if using jQuery 1.9.0, this code will throw an exception when
dealing with empty validation attributes.