Fix Ractor.store_if_absent

It in fact yields `nil`. Added `nil` in block parameter for testing.

core/ractor.rbs

@@ -603,7 +603,7 @@ class Ractor
   #       }
   #     }.map(&:value).uniq.size #=> 1 and f() is called only once
   #
-  def self.store_if_absent: (Symbol) { () -> untyped } -> untyped
+  def self.store_if_absent: [A] (Symbol) { (nil) -> A } -> A
 
   # <!--
   #   rdoc-file=ractor.rb

test/stdlib/Ractor_test.rb

@@ -107,6 +107,13 @@ def test_shareable?
                      Ractor, :shareable?, []
   end
 
+  def test_store_if_absent
+    assert_send_type(
+      "(Symbol) { (nil) -> true } -> true",
+      Ractor, :store_if_absent, :test_store_if_absent, &->(_x) { true }
+    )
+  end
+
   def test_yield
     Ractor.new(Ractor.current) { |r| loop { r.take } }