[vulkan] Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I75cc812c35076e6121998945e7fdb1ebe2462663
diff --git a/vulkan/libvulkan/driver.cpp b/vulkan/libvulkan/driver.cpp
index 56bc35e..4c2d223 100644
--- a/vulkan/libvulkan/driver.cpp
+++ b/vulkan/libvulkan/driver.cpp
@@ -984,7 +984,7 @@
     uint32_t icd_api_version;
     PFN_vkEnumerateInstanceVersion pfn_enumerate_instance_version =
         reinterpret_cast<PFN_vkEnumerateInstanceVersion>(
-            Hal::Device().GetInstanceProcAddr(NULL,
+            Hal::Device().GetInstanceProcAddr(nullptr,
                                               "vkEnumerateInstanceVersion"));
     if (!pfn_enumerate_instance_version) {
         icd_api_version = VK_API_VERSION_1_0;