use nullptr instead of NULL / 0 in rocksdbjni (facebook#12575)

Summary:
While I was trying to understand issue facebook#12503, I found this minor problem. Please have a look adamretter rhubner

Pull Request resolved: facebook#12575

Reviewed By: ajkr

Differential Revision: D57596055

Pulled By: cbi42

fbshipit-source-id: ee0860bdfbee9364cd30c23957b72a04da6acd45

java/rocksjni/options.cc

@@ -3917,7 +3917,7 @@ jlongArray Java_org_rocksdb_Options_tablePropertiesCollectorFactory(
  // exception thrown: OutOfMemoryError
  return nullptr;
  }
- jlong* buf = env->GetLongArrayElements(retVal, NULL);
+ jlong* buf = env->GetLongArrayElements(retVal, nullptr);
  if (buf == nullptr) {
  // exception thrown: OutOfMemoryError
  return nullptr;
@@ -3943,7 +3943,7 @@ void Java_org_rocksdb_Options_setTablePropertiesCollectorFactory(
  auto* opt = reinterpret_cast<ROCKSDB_NAMESPACE::Options*>(jhandle);
  const jsize size = env->GetArrayLength(j_factory_handles);
 
- jlong* buf = env->GetLongArrayElements(j_factory_handles, NULL);
+ jlong* buf = env->GetLongArrayElements(j_factory_handles, nullptr);
  if (buf == nullptr) {
  // exception thrown: OutOfMemoryError
  return;

java/rocksjni/portal.h

@@ -1689,7 +1689,7 @@ class JniUtil {
  } else if (env_rs == JNI_EDETACHED) {
  // current thread is not attached, attempt to attach
  const jint rs_attach =
- jvm->AttachCurrentThread(reinterpret_cast<void**>(&env), NULL);
+ jvm->AttachCurrentThread(reinterpret_cast<void**>(&env), nullptr);
  if (rs_attach == JNI_OK) {
  *attached = JNI_TRUE;
  return env;

java/rocksjni/ttl.cc

@@ -145,7 +145,7 @@ jlongArray Java_org_rocksdb_TtlDB_openCF(JNIEnv* env, jclass, jlong jopt_handle,
  return jresults;
  } else {
  ROCKSDB_NAMESPACE::RocksDBExceptionJni::ThrowNew(env, s);
- return NULL;
+ return nullptr;
  }
 }