Avoid running async queries on null relations

Calling `load_async` on a null relation was executing the fallback
`WHERE 1=0` query, since it doesn't go through `exec_queries`.

Since 2a90104989, `exec_queries` is
implemented in terms of `exec_main_query`, so we only need to override
that one method.

We can also stop assigning `@records`, as that now happens in `load`
since 847643a55a.

activerecord/lib/active_record/null_relation.rb

@@ -56,8 +56,8 @@ module ActiveRecord
     end
 
     private
-      def exec_queries
+      def exec_main_query(async: false)
-        @records = [].freeze
+        [].freeze
       end
   end
 end

activerecord/test/cases/null_relation_test.rb

@@ -28,6 +28,12 @@ class NullRelationTest < ActiveRecord::TestCase
     end
   end
 
+  def test_async_query_on_null_relation
+    assert_no_queries do
+      assert_equal [], Developer.none.load_async.load
+    end
+  end
+
   def test_none_chained_to_methods_firing_queries_straight_to_db
     assert_no_queries do
       assert_equal [],    Developer.none.pluck(:id, :name)