pybind/mgr/mirroring: Fix KeyError: 'directory_count' in daemon status

pybind/mgr/mirroring: Fix KeyError: 'directory_count' in daemon status

The directory_count key is missing in self.mgr.get_daemon_status() output json,
intermittently when there is a delay caused by m_listener.handle_mirroring_enabled() to update the
directory_count, which results in ServiceDaemon::update_status() creates a json with out 'directory_count' key/value.
But the mgr/mirroring -> daemon_status() always expects the 'directory_count' key to be present in the json returned by
self.mgr.get_daemon_status().

This issue occurs intermittently when we enable/disable mirroring and check the 'daemon status' in between.
This patch fixes this issue by setting a default value 0 for 'directory_count' in doemon_status().

Fixes: https://tracker.ceph.com/issues/65795
Signed-off-by: Jos Collin <jcollin@redhat.com>
(cherry picked from commit b78baa23e562742b8bdc5a75f82e3b6fbf55a8a5)