[wasm] Change default value of tables

The default value for table entries in WebAssembly tables is null when
the table gets allocated from WebAssembly, but when the table gets
allocated from JavaScript, the default value is undefined when the
table type is externref. With this CL V8 handles the JavaScript case
spec-compliant.

R=manoskouk@chromium.org

Bug: v8:12227
Change-Id: Ic8a1361629d8e5dfb59e2ee22a5e0ae0f6de936d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3162045


Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76892}