Return empty set early if no can rules are present.

Thanks dkubb!

lib/cancan/model_adapters/data_mapper_adapter.rb

@@ -17,6 +17,7 @@ module CanCan
       def database_records
         scope = @model_class.all(:conditions => ["0 = 1"])
         cans, cannots = @rules.partition { |r| r.base_behavior }
+        return scope if cans.empty?
         # apply unions first, then differences. this mean cannot overrides can
         cans.each    { |r| scope += @model_class.all(:conditions => r.conditions) }
         cannots.each { |r| scope -= @model_class.all(:conditions => r.conditions) }