Make sure respond_to? and method_missing works in abstract Railties

Those methods were raising an confusing exception when trying to call
`Rails::Railtie.respond_to?(:something)`. Now they just work.

Fixes #44761

railties/lib/rails/railtie.rb

@@ -214,13 +214,15 @@ def generate_railtie_name(string)
         end
 
         def respond_to_missing?(name, _)
+          return super if abstract_railtie?
+
           instance.respond_to?(name) || super
         end
 
         # If the class method does not have a method, then send the method call
         # to the Railtie instance.
         def method_missing(name, *args, &block)
-          if instance.respond_to?(name)
+          if !abstract_railtie? && instance.respond_to?(name)
             instance.public_send(name, *args, &block)
           else
             super

railties/test/railties/railtie_test.rb

@@ -21,7 +21,15 @@ def app
     end
 
     test "cannot instantiate a Railtie object" do
-      assert_raise(RuntimeError) { Rails::Railtie.new }
+      assert_raise(RuntimeError) { Rails::Railtie.send(:new) }
+    end
+
+    test "respond_to? works in the abstract railties" do
+      assert_not_respond_to Rails::Railtie, :something_nice
+    end
+
+    test "method_missing works in the abstract railties" do
+      assert_raise(NoMethodError) { Rails::Railtie.something_nice }
     end
 
     test "Railtie provides railtie_name" do