All votes deleted when $nid is NULL

There was a bug in another module that I was using that triggered UID 0 to be removed among with some other bad node data. This cause and empty node object to be deleted (which shouldn't be possible afaik) and when plus1 processed the node deletion removed all votes. Basically, when the select criteria for the content_id is NULL all votes will be returned, and in this case all votes were associated with the NULL content_id. Anyway, the simple patch here should prevent that from happening in the future, as unlikely as this should be to happen.