ValueError when showing resources whose "size" is not an int

Example: <http://www.publicdata.eu/dataset/pks-register-of-tender-documentation/resource/07554517-5171-4ff8-9ed3-fa5b6431a296>

It crashes because the resource's "size" is not an int: <http://publicdata.eu/api/3/action/resource_show?id=07554517-5171-4ff8-9ed3-fa5b6431a296>
```
File '/usr/lib/ckan/src/ckan/ckan/lib/helpers.py', line 1454 in format_resource_items
  value = formatters.localised_filesize(int(value))
ValueError: invalid literal for int() with base 10: 'None'
```