Restore an ability that reorder with first taking non-deterministic result

Actually, `first` taking non-deterministic result is considered a bug to
me. But changing the behavior should not be happened in rc2.

I've restored the behavior for 6.0, and then will deprecate that in 6.1.

Fixes #36802.

activerecord/lib/active_record/relation/query_methods.rb

@@ -372,7 +372,7 @@ def reorder(*args)
 
     # Same as #reorder but operates on relation in-place instead of copying.
     def reorder!(*args) # :nodoc:
-      preprocess_order_args(args)
+      preprocess_order_args(args) unless args.all?(&:blank?)
 
       self.reordering_value = true
       self.order_values = args

activerecord/test/cases/relations_test.rb

@@ -1737,6 +1737,20 @@ def test_reverse_order_with_reorder_nil_removes_the_order
     assert_nil relation.order_values.first
   end
 
+  def test_reorder_with_first
+    sql_log = capture_sql do
+      assert Post.order(:title).reorder(nil).first
+    end
+    assert sql_log.all? { |sql| !/order by/i.match?(sql) }, "ORDER BY was used in the query: #{sql_log}"
+  end
+
+  def test_reorder_with_take
+    sql_log = capture_sql do
+      assert Post.order(:title).reorder(nil).take
+    end
+    assert sql_log.all? { |sql| !/order by/i.match?(sql) }, "ORDER BY was used in the query: #{sql_log}"
+  end
+
   def test_presence
     topics = Topic.all