Currently the reconstruction of the shared heap object cache for testing
incorrectly includes the terminating undefined value. Unlike the RO cache
reconstruction, which does not change, the shared heap object cache may be
extended by serializing the live Isolate during testing, so it should skip the
original terminating undefined.

Bug: v8:12007, v8:12584
Change-Id: If73b865567ed7d5b658506e15b1dc8d14bd755d6
Cq-Include-Trybots: luci.v8.try:v8_linux64_gc_stress_custom_snapshot_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3421726
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78836}