test/openssl/test_pkey_*: skip tests for empty instances

These don't work and make no sense on OpenSSL 3.0, since PKey instances
are immutable once initialized.

Author: rhenium

test/openssl/test_pkey_dh.rb

@@ -10,7 +10,7 @@ def test_new_empty
     dh = OpenSSL::PKey::DH.new
     assert_equal nil, dh.p
     assert_equal nil, dh.priv_key
-  end
+  end if !openssl?(3, 0, 0)
 
   def test_new_generate
     # This test is slow

test/openssl/test_pkey_rsa.rb

@@ -167,7 +167,7 @@ def test_verify_empty_rsa
     assert_raise(OpenSSL::PKey::PKeyError, "[Bug #12783]") {
       rsa.verify("SHA1", "a", "b")
     }
-  end
+  end if !openssl?(3, 0, 0)
 
   def test_sign_verify_pss
     key = Fixtures.pkey("rsa1024")
@@ -390,7 +390,7 @@ def test_RSAPublicKey
     EOF
     key = OpenSSL::PKey::RSA.new(pem)
     assert_same_rsa rsa1024pub, key
-  end
+  end if !openssl?(3, 0, 0) # RSAPublicKey is not available on OpenSSL 3.0
 
   def test_PUBKEY
     rsa1024 = Fixtures.pkey("rsa1024")