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

xtoskrnl/mm/amd64/pagemap.cc

@@ -36,7 +36,7 @@ MM::PageMap::ClearPte(PHARDWARE_PTE PtePointer)
  * @param Address
  *        Specifies the virtual address for which to retrieve the corresponding P5E.
  *
- * @return This routine returns the address of the P5E, or NULL if LA57 is not enabled.
+ * @return This routine returns the address of the P5E, or NULLPTR if LA57 is not enabled.
  *
  * @since XT 1.0
  */

xtoskrnl/mm/hlpool.cc

@@ -176,13 +176,13 @@ MM::HardwarePool::MapHardwareMemory(IN PHYSICAL_ADDRESS PhysicalAddress,
     BaseAddress = HardwareHeapStart;
     MappedPages = 0;
     ReturnAddress = BaseAddress;
-    *VirtualAddress = NULL;
+    *VirtualAddress = NULLPTR;
 
     /* Iterate through all pages */
     while(MappedPages < PageCount)
     {
         /* Check if address overflows */
-        if(BaseAddress == NULL)
+        if(BaseAddress == NULLPTR)
         {
             /* Not enough free pages, return error */
             return STATUS_INSUFFICIENT_RESOURCES;