nooping on nil parameters

lib/arel/select_manager.rb

@@ -19,6 +19,8 @@ module Arel
     end
 
     def join relation, klass = Nodes::InnerJoin
+      return self unless relation
+
       case relation
       when String, Nodes::SqlLiteral
         raise if relation.blank?

spec/arel/select_manager_spec.rb

@@ -86,6 +86,11 @@ module Arel
                ON "users"."id" = "users_2"."id"
         }
       end
+
+      it 'noops on nil' do
+        manager   = Arel::SelectManager.new Table.engine
+        check manager.join(nil).should == manager
+      end
     end
 
     describe 'joins' do