1151490 - Repo group publish  fails with NoneType error

https://bugzilla.redhat.com/show_bug.cgi?id=1151490
If optional keys in the config are not specified then by default they are None
(except http/https).Validate_export_config() was not checking whether export_dir
(optional key) is None, and this led to the error when repo group publish was done
via pulp-admin.