Fix unscoping association scope on joins not to raise an error

[kamipo]

#29589 changed merging scope order to allow to unscope default scopes on
association scope (#29611), but that caused a regression #38811 that
accidentally allow join constraint which is required.

% bin/test test/cases/associations/has_many_associations_test.rb -n test_unscopes_the_default_scope_of_associated_model_when_used_with_include
Run options: -n test_unscopes_the_default_scope_of_associated_model_when_used_with_include --seed 32978

# Running:

E

Error:
HasManyAssociationsTest#test_unscopes_the_default_scope_of_associated_model_when_used_with_include:
NoMethodError: undefined method `children' for nil:NilClass
    ~/rails/activerecord/lib/active_record/associations/join_dependency/join_association.rb:39:in `block in join_constraints'
    ~/rails/activerecord/lib/active_record/associations/join_dependency/join_association.rb:30:in `reverse_each'
    ~/rails/activerecord/lib/active_record/associations/join_dependency/join_association.rb:30:in `with_index'
    ~/rails/activerecord/lib/active_record/associations/join_dependency/join_association.rb:30:in `join_constraints'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:171:in `make_constraints'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:196:in `block in walk'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:196:in `each'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:196:in `flat_map'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:196:in `walk'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:90:in `block in join_constraints'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:87:in `each'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:87:in `flat_map'
    ~/rails/activerecord/lib/active_record/associations/join_dependency.rb:87:in `join_constraints'
    ~/rails/activerecord/lib/active_record/relation/query_methods.rb:1226:in `build_join_query'
    ~/rails/activerecord/lib/active_record/relation/query_methods.rb:1211:in `build_joins'
    ~/rails/activerecord/lib/active_record/relation/query_methods.rb:1091:in `build_arel'
    ~/rails/activerecord/lib/active_record/relation/query_methods.rb:1063:in `arel'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:419:in `block in limited_ids_for'
    ~/rails/activerecord/lib/active_record/relation.rb:867:in `skip_query_cache_if_necessary'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:419:in `limited_ids_for'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:398:in `apply_join_dependency'
    ~/rails/activerecord/lib/active_record/relation.rb:839:in `block in exec_queries'
    ~/rails/activerecord/lib/active_record/relation.rb:867:in `skip_query_cache_if_necessary'
    ~/rails/activerecord/lib/active_record/relation.rb:834:in `exec_queries'
    ~/rails/activerecord/lib/active_record/relation.rb:639:in `load'
    ~/rails/activerecord/lib/active_record/relation.rb:250:in `records'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:508:in `find_take'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:98:in `take'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:458:in `find_one'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:442:in `find_with_ids'
    ~/rails/activerecord/lib/active_record/relation/finder_methods.rb:69:in `find'
    ~/rails/activerecord/test/cases/associations/has_many_associations_test.rb:2689:in `block in <class:HasManyAssociationsTest>'

bin/test test/cases/associations/has_many_associations_test.rb:2683

Required join constraint should not be allowed to unscoping.

Fixes #38811.

[alboyadjian]

Thank you!