[libsparse] Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: I43dae734817cae7a260ffc7afcd85fbd4451eddf
diff --git a/libsparse/sparse.cpp b/libsparse/sparse.cpp
index 6ff97b6..cb288c5 100644
--- a/libsparse/sparse.cpp
+++ b/libsparse/sparse.cpp
@@ -30,13 +30,13 @@
 struct sparse_file* sparse_file_new(unsigned int block_size, int64_t len) {
   struct sparse_file* s = reinterpret_cast<sparse_file*>(calloc(sizeof(struct sparse_file), 1));
   if (!s) {
-    return NULL;
+    return nullptr;
   }
 
   s->backed_block_list = backed_block_list_new(block_size);
   if (!s->backed_block_list) {
     free(s);
-    return NULL;
+    return nullptr;
   }
 
   s->block_size = block_size;
@@ -252,7 +252,7 @@
                                                   unsigned int len) {
   int64_t count = 0;
   struct output_file* out_counter;
-  struct backed_block* last_bb = NULL;
+  struct backed_block* last_bb = nullptr;
   struct backed_block* bb;
   struct backed_block* start;
   unsigned int last_block = 0;
@@ -270,7 +270,7 @@
   out_counter = output_file_open_callback(out_counter_write, &count, to->block_size, to->len, false,
                                           true, 0, false);
   if (!out_counter) {
-    return NULL;
+    return nullptr;
   }
 
   for (bb = start; bb; bb = backed_block_iter_next(bb)) {
@@ -281,7 +281,7 @@
     /* will call out_counter_write to update count */
     ret = sparse_file_write_block(out_counter, bb);
     if (ret) {
-      bb = NULL;
+      bb = nullptr;
       goto out;
     }
     if (file_len + count > len) {
@@ -330,13 +330,13 @@
     if (c < out_s_count) {
       out_s[c] = s;
     } else {
-      backed_block_list_move(s->backed_block_list, tmp->backed_block_list, NULL, NULL);
+      backed_block_list_move(s->backed_block_list, tmp->backed_block_list, nullptr, nullptr);
       sparse_file_destroy(s);
     }
     c++;
   } while (bb);
 
-  backed_block_list_move(tmp->backed_block_list, in_s->backed_block_list, NULL, NULL);
+  backed_block_list_move(tmp->backed_block_list, in_s->backed_block_list, nullptr, nullptr);
 
   sparse_file_destroy(tmp);