do not add fields to sql-query if they are empty

git-svn-id: https://dev.finn.de/svn/cockpit/branches/reporting_merge@1645 7926756e-e54e-46e6-9721-ed318f58905e

app/models/cost_query/filter/base.rb

@@ -100,7 +100,7 @@ module CostQuery::Filter
         arity   = operator.method(:modify).arity - 2
         values  = self.values || []
         values  = values[0, arity] if values and arity >= 0 and arity != values.size
-        operator.modify(query, field, *values)
+        operator.modify(query, field, *values) unless field.empty?
       end
     end
   end