export_database.pl - some very minor optimisations
export_database.pl - some very minor optimisations

turns out DateTime's constructor is really slow.
also, about half the time sanitize_field_content() was being called on an empty string, which still took time.

openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server