Empty `resolve_with` config array results in non-functional pipeline

[kgreunke]

At first glance this might be an obvious statement, however if you happen to end up with a config that falls through the below setup routine and ends up with an empty array, nothing works and nothing tells you why it's not working.

      if config.assets.resolve_with.nil?
        config.assets.resolve_with = []
        config.assets.resolve_with << :manifest if config.assets.digest && !config.assets.debug
        config.assets.resolve_with << :environment if config.assets.compile
      end

https://github.com/rails/sprockets-rails/blob/73e7351abff3506f6dca6b2da8abedfd5c7c0d77/lib/sprockets/railtie.rb#LL226C1-L230C10

I managed to hit this problem where someone set the assets debug flag in the initializer. When running in production mode the :manifest option gets kicked out because of the debug flag and the :environment option gets kicked out because the default option in rails is to not compile assets in production.

If :manifest and :environment are the only two available options for resolvers, and neither are picked up, something should throw a warning at some point that the pipeline isn't going to resolve any assets.

For search purposes this was causing ActionView::Template::Error (The asset "application.css" is not present in the asset pipeline.) based on whatever was the first pipeline asset to be rendered