Support nil in return value.

core/thread.rbs

@@ -495,7 +495,7 @@ class Thread < Object
   # native thread, it returns *nil*. If the Ruby implementation uses M:N thread
   # model, the ID may change depending on the timing.
   #
-  def native_thread_id: () -> Integer
+  def native_thread_id: () -> Integer?
 
   # <!--
   #   rdoc-file=thread.c

test/stdlib/Thread_test.rb

@@ -42,6 +42,8 @@ def test_native_thread_id
       "() -> Integer",
       Thread.current, :native_thread_id
     )
+
+    assert_send_type "() -> nil", Thread.new{}.join, :native_thread_id
   end
 
   def test_raise