Use nullptr in fboss/agent/hw/bcm/BcmEcmpUtils.cpp

Summary:
`nullptr` is preferable to `0` or `NULL`. Let's use it everywhere so we can enable `-Wzero-as-null-pointer-constant`.

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Reviewed By: jasmeetbagga

Differential Revision: D54835443

fbshipit-source-id: 032f9cbd82eb35204c8c3a0b710c1b245ad28417

fizz/extensions/delegatedcred/test/SelfDelegatedCredentialTest.cpp

@@ -59,8 +59,8 @@ class SelfDelegatedCredentialTest : public Test {
 
 #if FIZZ_OPENSSL_HAS_ED25519
   folly::ssl::EvpPkeyUniquePtr generateEd25519PrivKey() {
-    EVP_PKEY* pkey = NULL;
-    EVP_PKEY_CTX* pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_ED25519, NULL);
+    EVP_PKEY* pkey = nullptr;
+    EVP_PKEY_CTX* pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_ED25519, nullptr);
     EVP_PKEY_keygen_init(pctx);
     EVP_PKEY_keygen(pctx, &pkey);
     EVP_PKEY_CTX_free(pctx);