Using FMT_NULLPTR instead of literal 0

(cherry picked from commit 49ccb2e449)

test/format-test.cc

@@ -250,7 +250,7 @@ TEST(WriterTest, Allocator) {
   std::size_t size =
       static_cast<std::size_t>(1.5 * fmt::internal::INLINE_BUFFER_SIZE);
   std::vector<char> mem(size);
-  EXPECT_CALL(alloc, allocate(size)).WillOnce(testing::Return(&mem[0]));
+  EXPECT_CALL(alloc, allocate(size, 0)).WillOnce(testing::Return(&mem[0]));
   for (int i = 0; i < fmt::internal::INLINE_BUFFER_SIZE + 1; ++i)
     w << '*';
   EXPECT_CALL(alloc, deallocate(&mem[0], size));