mon/Monitor.cc: attempt to decode and extract keyring only if it is found.

mon/Monitor.cc: attempt to decode and extract keyring only if it is found.

Monitor::preinit wrongly assumes that keyring always exists. This worked
so far because the logic that followed made this error invisible.
Change the logic so no attempt is taken to decode empty bufferlist, and
in turn - to extract_save_mon_key from empty keyring.

Signed-off-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>