qa/cephfs: add code for when config is None in __init__

qa/cephfs: add code for when config is None in __init__

When tests are launched with kernel client using vstart_runner.py,
config is None and, therefore, the call "config.get()" leads to a crash.
Assigning self.rbytes None is important since leaving it undefined will
to lead a crash since the code executed later assumes that self.rbytes
is defined.

Fixes: https://tracker.ceph.com/issues/48147
Signed-off-by: Rishabh Dave <ridave@redhat.com>