osd/ReplicatedPG: do not choke on op-less flush OpContexts (from flush)

The agent initiates flush ops that don't have an OpRequest associated
with them.  Make reply_ctx skip the actual reply message instead of
crashing if the flush request gets canceled (e.g., due to a race with
a write).

Signed-off-by: Sage Weil <sage@inktank.com>