Allow setting config.hosts to nil

Prior to #46858, uses can set `config.hosts = nil` to allow all hosts.
But now it causes `NoMethodError` because the check uses only `empty?`.

Given that the HostAuthorization itself allows taking `nil` as hosts,
I think this breaking change is not intentional. Therefore this commit
fixes the issue by also checking if the value is `nil`.

railties/lib/rails/application/default_middleware_stack.rb

@@ -13,7 +13,7 @@ module Rails
 
       def build_stack
         ActionDispatch::MiddlewareStack.new do |middleware|
-          unless config.hosts.empty?
+          unless config.hosts.nil? || config.hosts.empty?
             middleware.use ::ActionDispatch::HostAuthorization, config.hosts, **config.host_authorization
           end
 

railties/test/application/middleware_test.rb

@@ -154,6 +154,13 @@ module ApplicationTests
       assert_not_includes middleware, "ActionDispatch::HostAuthorization"
     end
 
+    test "ActionDispatch::HostAuthorization is not included if hosts is set to nil" do
+      add_to_config "config.hosts = nil"
+      boot!
+
+      assert_not_includes middleware, "ActionDispatch::HostAuthorization"
+    end
+
     test "Rack::Cache is not included by default" do
       boot!