Fix deprecated use of 0/NULL in featured/util/ClassName.h + 3

Summary:
`nullptr` is typesafe. `0` and `NULL` are not. In the future, only `nullptr` will be allowed.

This diff helps us embrace the future _now_ in service of enabling `-Wzero-as-null-pointer-constant`.

Reviewed By: meyering

Differential Revision: D54163048

fbshipit-source-id: b7df83f52abba3ae9f628a5fc8843f073c33a4d6

fizz/crypto/signature/Signature.cpp

@@ -91,7 +91,8 @@ std::unique_ptr<folly::IOBuf> edSign(
     throw std::runtime_error(
         to<std::string>("Could not allocate EVP_MD_CTX", getOpenSSLError()));
   }
-  if (EVP_DigestSignInit(mdCtx.get(), NULL, NULL, NULL, pkey.get()) != 1) {
+  if (EVP_DigestSignInit(mdCtx.get(), nullptr, nullptr, nullptr, pkey.get()) !=
+      1) {
     throw std::runtime_error("Could not initialize digest signature");
   }
   auto out = folly::IOBuf::create(EVP_PKEY_size(pkey.get()));
@@ -122,7 +123,8 @@ void edVerify(
     throw std::runtime_error(
         to<std::string>("Could not allocate EVP_MD_CTX", getOpenSSLError()));
   }
-  if (EVP_DigestVerifyInit(mdCtx.get(), NULL, NULL, NULL, pkey.get()) != 1) {
+  if (EVP_DigestVerifyInit(
+          mdCtx.get(), nullptr, nullptr, nullptr, pkey.get()) != 1) {
     throw std::runtime_error("Could not initialize digest signature");
   }