Filter out NULL values from the $items array passed to the formatter.

Working on PIFT tonight, I discovered that http://git7site.devdrupal.org/node/1947880 was throwing all sorts of notices for extended_file_field ... a bit of investigation showed that the node was returning an empty element within the field_issue_files field.

I'm not sure what's caused that, but adding a simple array_filter() cleared it up.

Based on the notices, I suspect the same treatment will need to be applied to nodechanges.