have an actual `NullColumn` object and update docs accordingly.

Follow up to #15438 and #15502.

/cc @sgrif

activerecord/lib/active_record/attribute_methods.rb

@@ -195,8 +195,9 @@ def attribute_names
           end
       end
 
-      # Returns the column object for the named attribute. Returns +nil+ if the
-      # named attribute not exists.
+      # Returns the column object for the named attribute.
+      # Returns a +ActiveRecord::ConnectionAdapters::NullColumn+ if the
+      # named attribute does not exist.
       #
       #   class Person < ActiveRecord::Base
       #   end
@@ -206,11 +207,11 @@ def attribute_names
       #   # => #<ActiveRecord::ConnectionAdapters::SQLite3Column:0x007ff4ab083980 @name="name", @sql_type="varchar(255)", @null=true, ...>
       #
       #   person.column_for_attribute(:nothing)
-      #   # => #<ActiveRecord::ConnectionAdapters::Column:0xXXX @name=nil, @sql_type=nil, @cast_type=#<Type::Value>, ...>
+      #   # => #<ActiveRecord::ConnectionAdapters::NullColumn:0xXXX @name=nil, @sql_type=nil, @cast_type=#<Type::Value>, ...>
       def column_for_attribute(name)
         name = name.to_s
         columns_hash.fetch(name) do
-          ConnectionAdapters::Column.new(name, nil, Type::Value.new)
+          ConnectionAdapters::NullColumn.new(name)
         end
       end
     end

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -14,7 +14,10 @@ module ActiveRecord
   module ConnectionAdapters # :nodoc:
     extend ActiveSupport::Autoload
 
-    autoload :Column
+    autoload_at 'active_record/connection_adapters/column' do
+      autoload :Column
+      autoload :NullColumn
+    end
     autoload :ConnectionSpecification
 
     autoload_at 'active_record/connection_adapters/abstract/schema_definitions' do

activerecord/lib/active_record/connection_adapters/column.rb

@@ -55,6 +55,12 @@ def extract_default(default)
         type_cast(default)
       end
     end
+
+    class NullColumn < Column
+      def initialize(name)
+        super name, nil, Type::Value.new
+      end
+    end
   end
   # :startdoc:
 end

activerecord/test/cases/reflection_test.rb

@@ -82,6 +82,7 @@ def test_integer_columns
 
   def test_non_existent_columns_return_null_object
     column = @first.column_for_attribute("attribute_that_doesnt_exist")
+    assert_instance_of ActiveRecord::ConnectionAdapters::NullColumn, column
     assert_equal "attribute_that_doesnt_exist", column.name
     assert_equal nil, column.sql_type
     assert_equal nil, column.type