Fix Status page error 500

Traceback (most recent call last):
  File "/opt/minarca-server/lib/python3.7/site-packages/cherrypy/_cprequest.py", line 638, in respond
    self._do_respond(path_info)
  File "/opt/minarca-server/lib/python3.7/site-packages/cherrypy/_cprequest.py", line 697, in _do_respond
    response.body = self.handler()
  File "/opt/minarca-server/lib/python3.7/site-packages/cherrypy/lib/encoding.py", line 223, in __call__
    self.body = self.oldhandler(*args, **kwargs)
  File "/opt/minarca-server/lib/python3.7/site-packages/cherrypy/_cpdispatch.py", line 54, in __call__
    return self.callable(*self.args, **self.kwargs)
  File "/opt/minarca-server/lib/python3.7/site-packages/rdiffweb/controller/page_status.py", line 76, in default
    params = self._data(limit)
  File "/opt/minarca-server/lib/python3.7/site-packages/rdiffweb/controller/page_status.py", line 60, in _data
    success[key] += 1
KeyError: '2022-03-27'