Remove ActionDispatch::Flash::NullFlash

After #42167, all apps (except api only ones) have access to the flash
module. If the session store is disabled, then an empty flash object
is used.

This patch also prevents the flash from being committed to the session
if this is not enabled.

actionpack/lib/action_controller/metal/request_forgery_protection.rb

@@ -199,6 +199,10 @@ module ActionController #:nodoc:
             def exists?
               true
             end
+
+            def enabled?
+              false
+            end
           end
 
           class NullCookieJar < ActionDispatch::Cookies::CookieJar #:nodoc:

actionpack/lib/action_controller/test_case.rb

@@ -211,6 +211,10 @@ module ActionController
       @data.fetch(key.to_s, *args, &block)
     end
 
+    def enabled?
+      true
+    end
+
     private
       def load!
         @id

actionpack/lib/action_dispatch/middleware/flash.rb

@@ -45,7 +45,6 @@ module ActionDispatch
       # read a notice you put there or <tt>flash["notice"] = "hello"</tt>
       # to put a new one.
       def flash
-        return Flash::NullFlash unless session.respond_to?(:loaded?)
         flash = flash_hash
         return flash if flash
         self.flash = Flash::FlashHash.from_session_value(session["flash"])
@@ -60,16 +59,14 @@ module ActionDispatch
       end
 
       def commit_flash # :nodoc:
-        session    = self.session || {}
-        flash_hash = self.flash_hash
+        return unless session.enabled?
 
         if flash_hash && (flash_hash.present? || session.key?("flash"))
           session["flash"] = flash_hash.to_session_value
           self.flash = flash_hash.dup
         end
 
-        if (!session.respond_to?(:loaded?) || session.loaded?) && # reset_session uses {}, which doesn't implement #loaded?
-            session.key?("flash") && session["flash"].nil?
+        if session.loaded? && session.key?("flash") && session["flash"].nil?
           session.delete("flash")
         end
       end
@@ -80,20 +77,6 @@ module ActionDispatch
       end
     end
 
-    module NullFlash #:nodoc:
-      class << self
-        def []=(k, v); end
-
-        def [](k); end
-
-        def alert=(message); end
-
-        def notice=(message); end
-
-        def empty?; end
-      end
-    end
-
     class FlashNow #:nodoc:
       attr_accessor :flash