Fix OpenSSL::PKey::{RSA,DSA,DH}#params return type

Follow up to ruby/openssl#774

Author: ksss

stdlib/openssl/0/openssl.rbs

@@ -5648,7 +5648,7 @@ module OpenSSL
       # Stores all parameters of key to the hash INSECURE: PRIVATE INFORMATIONS CAN
       # LEAK OUT!!! Don't use :-)) (I's up to you)
       #
-      def params: () -> Hash[String, BN]
+      def params: () -> Hash[String, BN?]
 
       # <!--
       #   rdoc-file=ext/openssl/ossl_pkey_dh.c
@@ -5942,7 +5942,7 @@ module OpenSSL
       # Stores all parameters of key to the hash INSECURE: PRIVATE INFORMATIONS CAN
       # LEAK OUT!!! Don't use :-)) (I's up to you)
       #
-      def params: () -> Hash[String, BN]
+      def params: () -> Hash[String, BN?]
 
       def priv_key: () -> BN
 
@@ -7220,7 +7220,7 @@ module OpenSSL
       #
       # Don't use :-)) (It's up to you)
       #
-      def params: () -> Hash[String, BN]
+      def params: () -> Hash[String, BN?]
 
       # <!--
       #   rdoc-file=ext/openssl/ossl_pkey_rsa.c

test/stdlib/OpenSSL_test.rb

@@ -699,7 +699,7 @@ def test_export
   end
 
   def test_params
-    assert_send_type "() -> Hash[String, OpenSSL::BN]",
+    assert_send_type "() -> Hash[String, OpenSSL::BN?]",
       pkey, :params
   end
 
@@ -916,6 +916,11 @@ def test_sign_and_verify
       pkey, :verify, digest, sig, data
   end
 
+  def test_params
+    assert_send_type "() -> Hash[String, OpenSSL::BN]",
+      pkey, :params
+  end
+
   private
 
   def pkey