Association loading isn't to be affected by null relation scoping

Follow up of rails#35868.

Closes rails#19349.

Author: kamipo

activerecord/lib/active_record/relation.rb

@@ -689,7 +689,7 @@ def inspect
     end
 
     def empty_scope? # :nodoc:
-      @values == klass.unscoped.values
+      !null_relation? && @values == klass.unscoped.values
     end
 
     def has_limit_or_offset? # :nodoc:
@@ -721,6 +721,10 @@ def load_records(records)
         @loaded = true
       end
 
+      def null_relation? # :nodoc:
+        is_a?(NullRelation)
+      end
+
     private
       def already_in_scope?
         @delegate_to_klass && begin
@@ -770,7 +774,7 @@ def exec_queries(&block)
           @records =
             if eager_loading?
               apply_join_dependency do |relation, join_dependency|
-                if ActiveRecord::NullRelation === relation
+                if relation.null_relation?
                   []
                 else
                   relation = join_dependency.apply_column_aliases(relation)

activerecord/lib/active_record/scoping/named.rb

@@ -58,7 +58,7 @@ def default_scoped(scope = relation) # :nodoc:
         end
 
         def default_extensions # :nodoc:
-          if scope = current_scope || build_default_scope
+          if scope = scope_for_association || build_default_scope
             scope.extensions
           else
             []

activerecord/test/cases/scoping/relation_scoping_test.rb

@@ -421,6 +421,18 @@ def test_nested_scope_finder
     end
   end
 
+  def test_none_scoping
+    Comment.none.scoping do
+      assert_equal 2, @welcome.comments.count
+      assert_equal "a comment...", @welcome.comments.what_are_you
+    end
+
+    Comment.where("1=1").scoping do
+      assert_equal 2, @welcome.comments.count
+      assert_equal "a comment...", @welcome.comments.what_are_you
+    end
+  end
+
   def test_should_maintain_default_scope_on_associations
     magician = BadReference.find(1)
     assert_equal [magician], people(:michael).bad_references
@@ -461,4 +473,16 @@ def test_nested_scope_finder
       assert_equal "a category...", @welcome.categories.what_are_you
     end
   end
+
+  def test_none_scoping
+    Category.none.scoping do
+      assert_equal 2, @welcome.categories.count
+      assert_equal "a category...", @welcome.categories.what_are_you
+    end
+
+    Category.where("1=1").scoping do
+      assert_equal 2, @welcome.categories.count
+      assert_equal "a category...", @welcome.categories.what_are_you
+    end
+  end
 end