Fixed forwarding empty multi-select fields.

[0xLBF]

select2 doesn't include a field in the POST if multiple selections are allowed but none were selected by the user. The processor normally forwards the submissions to a filter before saving, but the submission_data doesn't include such empty multi-select fields. The value of such an empty field, as returned by get_field_data, is null so update_post_meta still doesn't save it, but this patch allows a custom filter to handle it more easily (e.g converting null values to empty strings, arrays to comma separated lists etc.).