Ensure extra_disk_data is skipped if nil
Ensure extra_disk_data is skipped if nil

This commit skips over adding any extra_disk_data to the storage
controller data structure in case it's nil.

sni/vagrant,chrisroberts/vagrant,sni/vagrant,marxarelli/vagrant,marxarelli/vagrant,sni/vagrant,chrisroberts/vagrant,marxarelli/vagrant,mitchellh/vagrant,sni/vagrant,mitchellh/vagrant,chrisroberts/vagrant,mitchellh/vagrant,mitchellh/vagrant,chrisroberts/vagrant,marxarelli/vagrant