Fix 'Undefined offset: 0' error for null values.

When there were nulls in the data, and you were using $extract, it would give you an error:

Undefined offset: 0 [APP/Plugin/CsvView/View/CsvView.php, line 218]

This patch fixes that error, and shows null values as 'NULL' in the exported CSV. I debated wether to show null's as 'NULL' or just '' but I think 'NULL' is less ambiguous and avoids any chance of confusion. Happy to change if you think differently...

Signed-off-by: Joshua Paling <joshua.paling@gmail.com>