Add the empty preprocessed_data/*/py/path_contexts.csv files exception handling 

Currently rises: 

```
File "code2vec/preprocess.py", line 135, in <module>
    max_contexts=int(args.max_contexts))
  File "code2vec/preprocess.py", line 69, in process_file
    print('Average total contexts: ' + str(float(sum_total) / total))
ZeroDivisionError: float division by zero
```