Replace all occurrences of NULL with NULLPTR for unified C and C++ null pointer handling

xtoskrnl/ke/amd64/krnlinit.cc

@@ -99,7 +99,8 @@ KE::KernelInit::StartKernel(VOID)
     CurrentProcess->Quantum = MAXCHAR;
 
     /* Initialize Idle thread */
-    KThread::InitializeThread(CurrentProcess, CurrentThread, nullptr, nullptr, nullptr, nullptr, nullptr, AR::ProcSup::GetBootStack(), TRUE);
+    KThread::InitializeThread(CurrentProcess, CurrentThread, NULLPTR, NULLPTR, NULLPTR,
+                              NULLPTR, NULLPTR, AR::ProcSup::GetBootStack(), TRUE);
     CurrentThread->NextProcessor = Prcb->CpuNumber;
     CurrentThread->Priority = THREAD_HIGH_PRIORITY;
     CurrentThread->State = Running;

xtoskrnl/ke/amd64/kthread.cc

@@ -99,7 +99,7 @@ KE::KThread::InitializeThreadContext(IN PKTHREAD Thread,
         Thread->NpxState = NPX_STATE_UNUSED;
 
         /* Set thread start address */
-        ThreadFrame->StartFrame.Return = (ULONG64)NULL;
+        ThreadFrame->StartFrame.Return = (ULONG64)NULLPTR;
     }
 
     /* Initialize thread startup information */