mon: AuthMonitor: don't try to auth import a null buffer

mon: AuthMonitor: don't try to auth import a null buffer

Hangs result if 'ceph auth import' is attempted without -i.
Check for this case and return error status.  Also,
update auth import help to more-clearly indicate that "input"
means "-i <file>".

Fixes: #4599
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>