PYCBC-199, PYCBC-203: Don't send invalid JSON in POST when no keys

We might still want to consider what to do in this case, but sending
'None' is certainly wrong :)

Change-Id: I84775f7380846f336cc1f213a18619e7128ea023
Reviewed-on: http://review.couchbase.org/29698
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>