Made Image descriptions nullable

org.jclouds.compute.domain.Image has "name" and "description" properties. "name" is Nullable but "description" isn't. This seems to be a little inconsistent, why should an Image description String be so strict about its contents if the name is completely ambivalent?

While this seems to be a pretty trivial matter the "openstack-nova" provider has no concept of an Image description, only a name. This is worked around by assigning the "openstack-nova" name to both the JClouds Image name and description. I'm experiencing an issue when the "openstack-nova" has a null name and tries to assign it to the currently NOT nullable description.

I hope this makes sense :)