PYCBC-248: Return from stats handler if operation is complete
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 <e7d42768707bf23038325b0b68d4b577e5f6064a@haskalah.org>
Reviewed-by: Volker Mische <fb414f8ac0dbbf87663550ae4ef5fc95b1041941@gmail.com>

couchbase/couchbase-python-client,mnunberg/couchbase-python-client,couchbase/couchbase-python-client,mnunberg/couchbase-python-client