ensure_inheritance condition

`subclasses` was added as part of Ruby 3.1
https://ruby-doc.org/core-3.1.0/Class.html#method-i-subclasses

Add a `respond_to?` to `base` to ensure that Ruby 3.0 will continue to
work as expected.

#511 (comment)

lib/statesman/adapters/active_record_queries.rb

@@ -39,7 +39,7 @@ def initialize(**args)
         end
 
         def included(base)
-          ensure_inheritance(base) unless base.subclasses.none?
+          ensure_inheritance(base) if base.respond_to?(:subclasses) && base.subclasses.any?
 
           query_builder = QueryBuilder.new(base, **@args)