Fix TypeError bug in DictReader when file is empty

Instantiating a `unicodecsv.DictReader` on an empty file currently raises a `TypeError: 'NoneType' object is not iterable` exception. For example:

```python
import unicodecsv

with open('empty_file.csv') as f:
    reader = unicodecsv.DictReader(f)  # raises TypeError
```

This commit fixes the bug. I'm happy to respond to feedback.

Thanks!