fix for feature results when there are no features found in a django app from harvest

```
could not find features at ./env/local/lib/python2.7/site-packages/django_extensions/features
> /home/gxx/projects/lettuce/lettuce/lettuce/core.py(1343)summarize_all()
   1342         import ipdb;ipdb.set_trace()
-> 1343         for partial_result in self.total_results:
   1344             self.features_ran_overall += len(partial_result.feature_results)

ipdb> self.total_results
[None, None, <lettuce.core.TotalResult object at 0x317add0>, None]
```

This causes an error as the following code is not expected ```None```.
Apologies, I don't have time to make the tests.