NCBC-306: .NET GetJSON operation throws null reference exception

Add support for null values persisted for a key via the
CouchbaseClientExtensions.GetJson(…) method. This method will no 
longer thrown a NullReferenceException when the value store for a 
key is null and instead simply return null.

Change-Id: I345cb7e33b5e9f33ea22cb205de84475abf3d35d
Reviewed-on: http://review.couchbase.org/29840
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>