We are always passing a valid buffer to WasmMemoryObject::New, hence
make it a Handle instead of MaybeHandle, and remove the code to allocate
an empty buffer if none was passed.

R=ahaas@chromium.org

Bug: v8:10949
Change-Id: Ic10dfda741773a8f3d3895f81a83048cfe0eb293
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3702805
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81172}