Pluck on NullRelation accepts a list of columns

`pluck` was updated to accept a list of columns, but the `NullRelation`
was never updated to match that signature. As a result, calling `pluck`
on a `NullRelation` results in an `ArgumentError`.

activerecord/CHANGELOG.md

+*   `NullRelation#pluck` takes a list of columns
+
+    The method signature in `NullRelation` was updated to mimic that in
+    `Calculations`.
+
+    *Derek Prior*
+
 *   `scope_chain` should not be mutated for other reflections.
 
     Currently `scope_chain` uses same array for building different

activerecord/lib/active_record/null_relation.rb

@@ -6,7 +6,7 @@ def exec_queries
       @records = []
     end
 
-    def pluck(_column_name)
+    def pluck(*column_names)
       []
     end
 

activerecord/test/cases/relations_test.rb

@@ -274,7 +274,7 @@ def test_none_chainable_to_existing_scope_extension_method
 
   def test_none_chained_to_methods_firing_queries_straight_to_db
     assert_no_queries do
-      assert_equal [],    Developer.none.pluck(:id) # => uses select_all
+      assert_equal [],    Developer.none.pluck(:id, :name)
       assert_equal 0,     Developer.none.delete_all
       assert_equal 0,     Developer.none.update_all(:name => 'David')
       assert_equal 0,     Developer.none.delete(1)