This avoids a warning by clang-tidy's modernize-use-nullptr.

Bug: v8:10488
Change-Id: I2820c7e5223101c5e3f7122d2a6a3526d36851e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2218063Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68065}