Be more resiliant (to things like NullHandler) when looking through log handlers.

On a clean `aminator` install, I was getting the following error:

```
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "aminator/cli.py", line 51, in run
    sys.exit(Aminator(debug=args.debug).aminate())
  File "aminator/core.py", line 52, in __init__
    configure_datetime_logfile(self.config, 'aminator')
  File "aminator/config.py", line 174, in configure_datetime_logfile
    if h.name == handler:
AttributeError: NullHandler instance has no attribute 'name'
```

I presume this is due to some missing configuration, but in any case the code should be able to deal with this without blowing up. :-)