Add guard for calling finalize on NullSession

rails#41372 Added
`ActiveRecord::AsynchronousQueriesTracker::NullSession` which replaced a
use of `nil` in `AsynchronousQueriesTracker`.  This commit changes the
`finalize_session` method to match that change from `nil` and properly
handle cases where it is called with a `NullSession` present.

Author: leequarella

activerecord/lib/active_record/asynchronous_queries_tracker.rb

@@ -50,7 +50,7 @@ def start_session
     end
 
     def finalize_session
-      @current_session&.finalize
+      @current_session.finalize if @current_session.respond_to?(:finalize)
       @current_session = NullSession
     end
   end

activerecord/test/cases/adapter_test.rb

@@ -408,6 +408,20 @@ def test_async_query_foreground_fallback
     ensure
       ActiveSupport::Notifications.unsubscribe(subscriber) if subscriber
     end
+
+    def test_async_query_finalize_with_null_session
+      assert_nothing_raised do
+        @connection.select_all "SELECT * FROM posts", async: true
+      end
+
+      @connection.transaction do
+        assert_raises AsynchronousQueryInsideTransactionError do
+          @connection.select_all "SELECT * FROM posts", async: true
+        end
+      end
+    ensure
+      ActiveRecord::Base.asynchronous_queries_tracker.finalize_session
+    end
   end
 
   class AsynchronousQueriesTest < ActiveRecord::TestCase