Don't cast nil to string in pg enums

Fixes #19389.

activerecord/lib/active_record/connection_adapters/postgresql/oid/enum.rb

@@ -7,7 +7,9 @@ module ActiveRecord
             :enum
           end
 
-          def cast(value)
+          private
+
+          def cast_value(value)
             value.to_s
           end
         end

activerecord/test/cases/adapters/postgresql/enum_test.rb

@@ -80,4 +80,12 @@ class PostgresqlEnumTest < ActiveRecord::TestCase
 
     assert_equal "happy", enum.current_mood
   end
+
+  def test_assigning_enum_to_nil
+    model = PostgresqlEnum.new(current_mood: nil)
+
+    assert_nil model.current_mood
+    assert model.save
+    assert_nil model.reload.current_mood
+  end
 end