PYCBC-248: Return from stats handler if operation is complete

Previously this was not exiting the callback handler if we received the
magic 'NULL callback'

Change-Id: I9e9aa6fc2acbb5272cb2ecaf0309126561733e7b
Reviewed-on: http://review.couchbase.org/38152
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>