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

xtoskrnl/ar/amd64/cpufunc.cc

@@ -709,7 +709,7 @@ AR::CpuFunc::StoreSegment(IN USHORT Segment,
                              : "=r" (*(PUINT)Destination));
             break;
         default:
-            Destination = NULL;
+            Destination = NULLPTR;
             break;
     }
 }

xtoskrnl/ar/amd64/procsup.cc

@@ -307,7 +307,7 @@ AR::ProcSup::InitializeProcessorBlock(OUT PKPROCESSOR_BLOCK ProcessorBlock,
     ProcessorBlock->Prcb.CurrentThread = &(KE::KThread::GetInitialThread())->ThreadControlBlock;
     ProcessorBlock->Prcb.CurrentThread->ApcState.Process = &(KE::KProcess::GetInitialProcess())->ProcessControlBlock;
     ProcessorBlock->Prcb.IdleThread = &(KE::KThread::GetInitialThread())->ThreadControlBlock;
-    ProcessorBlock->Prcb.NextThread = nullptr;
+    ProcessorBlock->Prcb.NextThread = NULLPTR;
 
     /* Set initial MXCSR register value */
     ProcessorBlock->Prcb.MxCsr = INITIAL_MXCSR;