[GH-443] Allow 'nullable' attribute to be used during XML export

This issue is created automatically through a Github pull request on behalf of cas87:

Url: https://github.com/doctrine/doctrine2/pull/443

Message:

Allows 'nullable' attribute to be used during XML export, something which already worked for me in YamlExport. This addition saved me a lot of time during development; one less difference to deal with between the YAML and XML approach.

Don't know why this was missing, maybe it's me who is missing something, so let me know