Replace some nullptrs with VK_NULL_HANDLE

src/gpu/vk/GrVkImage.h

Index: src/gpu/vk/GrVkImage.h
diff --git a/src/gpu/vk/GrVkImage.h b/src/gpu/vk/GrVkImage.h
index 3467a61f2bb735a9ec5ae89dcf53376c8e344490..eb0f7bf4fe99976f1a2bf8cff7639df2c4f5a324 100644
--- a/src/gpu/vk/GrVkImage.h
+++ b/src/gpu/vk/GrVkImage.h
@@ -29,7 +29,7 @@ public:
         VkDeviceMemory fAlloc;
         Flags          fFlags;
 
-        Resource() : INHERITED(), fImage(nullptr), fAlloc(nullptr), fFlags(kNo_Flags) {}
+        Resource() : INHERITED(), fImage(VK_NULL_HANDLE), fAlloc(VK_NULL_HANDLE), fFlags(kNo_Flags) {}
 
         Resource(VkImage image, VkDeviceMemory alloc, Flags flags)
             : fImage(image), fAlloc(alloc), fFlags(flags) {}