Allow returning of None in CSVFeedSpider

to support ignoring rows (header/footer and such).

Example using CSVFeedSpider:
```py
def parse_row(self, response, row):
    if row['COL1'] == 'FIRST-COLUMN':
        self.log('Ignoring CSV header row')
        return

    [... continue here]
```

At this time, the Spider will barf when returning nothing/`None`.