Delegate primary_key to super in abstract class

[dlwr]

Related Issues and PRs:

• Fix issue to avoid errors when table_name is nil #218
• Rails 7.1 - error when multi_tenant defined in abstract class #230

When I submitted PR #218, my approach was rather symptomatic. Now, I have a better understanding.

• Rails 7.1 has added support for composite_primary_key as mentioned in the Ruby on Rails 7.1 Release Notes.
• This change ensures that primary_key is checked for all ActiveRecord classes, including abstract classes.
• The .primary_key method implemented in this gem throws an exception for abstract classes because table_name is nil. I added a nil guard to make AbstractClass.primary_key return nil (Fix issue to avoid errors when table_name is nil #218).

However, this approach seems inappropriate. As indicated by the following link, ActiveRecord is designed to have AbstractClass.primary_key return 'id' and not nil:
https://github.com/rails/rails/blob/de4d8744744acab2dd9db0683ccf784ea45810b2/activerecord/lib/active_record/attribute_methods/primary_key.rb#L106-L116

The value of AbstractClass.primary_key is not the concern of this PR, so I have updated the implementation to delegate to super in such cases.

If there are any other approaches or suggestions, please let me know and I will implement them accordingly.