Use default label if object label would be blank

This is sort of similar to how the ```#object_infos``` method works. If the object_label is present, use that, but otherwise use the default method provided by rails admin.  Right now, if the object_label is blank, it prevents a lot of behavior, such as being able to click on the link for the associated object to view the details.

I thought about being a little more specific with this, iterating through the object_label_methods and returning the first value that was present, but decided against it for performance reasons.