Handle templates returned directly by resolvers

[saidM]

Rails main changed ActionView::Resolver#find to return a single template.

ResolverDecorator#nullify_templates only handled collections, so the template was returned unchanged and the real view could render when render_views was disabled.

This updates the resolver decorator to handle both single templates and collections.

Fixes #2905

Testing

• Added a regression spec for a resolver returning a single template
• Ran bundle exec rspec spec/rspec/rails/view_rendering_spec.rb
• Ran RuboCop

[pirj]

Thank you!

[saidM]

hey @JonRowe, could you please have a look?