Don't crash if getStats returns None

Hi!

This patch fixes the following error:

Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/opt/exaproxy/lib/exaproxy/util/debug.py", line 71, in <module>
    execfile(sys.argv[0])
  File "/opt/exaproxy/lib/exaproxy/application.py", line 317, in <module>
    main()
  File "/opt/exaproxy/lib/exaproxy/application.py", line 280, in main
    Supervisor(configuration).run()
  File "/opt/exaproxy/lib/exaproxy/supervisor.py", line 300, in run
    self.monitor.second()
  File "/opt/exaproxy/lib/exaproxy/monitor.py", line 146, in second
    self.seconds.append(self.statistics())
  File "/opt/exaproxy/lib/exaproxy/monitor.py", line 122, in statistics
    [redirector_stats] = redirector.getStats()
TypeError: 'NoneType' object is not iterable