Update simple_backend.py

As per #657 reduce can be called on empty list 'queries' 
This however will only  happen when a model unsuitable for the simple_backend is specified.

```syntax:python
    for field in model._meta._fields():
        if hasattr(field, 'related'):
            continue
        if not field.get_internal_type() in ('TextField', 'CharField', 'SlugField'):
            continue
```
```syntax:python

Traceback:
File "/home/ben/.virtualenvs/cc/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  115.                         response = callback(request, *callback_args, **callback_kwargs)
File "/home/ben/.virtualenvs/cc/local/lib/python2.7/site-packages/haystack/views.py" in __call__
  50.         return self.create_response()
File "/home/ben/.virtualenvs/cc/local/lib/python2.7/site-packages/haystack/views.py" in create_response
  130.         (paginator, page) = self.build_page()
File "/home/ben/.virtualenvs/cc/local/lib/python2.7/site-packages/haystack/views.py" in build_page
  107.         self.results[start_offset:start_offset + self.results_per_page]
File "/home/ben/.virtualenvs/cc/local/lib/python2.7/site-packages/haystack/query.py" in __getitem__
  268.                 self._fill_cache(start, bound)
File "/home/ben/.virtualenvs/cc/local/lib/python2.7/site-packages/haystack/query.py" in _fill_cache
  166.         results = self.query.get_results(**kwargs)
File "/home/ben/.virtualenvs/cc/local/lib/python2.7/site-packages/haystack/backends/__init__.py" in get_results
  644.                 self.run(**kwargs)
File "/home/ben/.virtualenvs/cc/local/lib/python2.7/site-packages/haystack/backends/__init__.py" in run
  562.         results = self.backend.search(final_query, **search_kwargs)
File "/home/ben/.virtualenvs/cc/local/lib/python2.7/site-packages/haystack/backends/__init__.py" in wrapper
  34.             return func(obj, query_string, *args, **kwargs)
File "/home/ben/.virtualenvs/cc/local/lib/python2.7/site-packages/haystack/backends/simple_backend.py" in search
  76.                         qs = model.objects.filter(six.moves.reduce(lambda x, y: x|y, queries))

Exception Type: TypeError at /search/
Exception Value: reduce() of empty sequence with no initial value
```