This is a reland of 453e1a3b

Added canonical "empty" arrays to ScannerStream::ForTesting, for the
zero-length nullptr data case.

Original change's description:
> [offthread] Add SFI support to OffThreadFactory
>
> Add support for off-thread SharedFunctionInfo allocation, which
> includes UncompiledData and PreparseData allocation.
>
> Bug: chromium:1011762
> Change-Id: Ia10f9ce762c7d7eb1108b9e71da75131dce919b7
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2050393
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#66246}

TBR=ulan@chromium.org

Bug: chromium:1011762
Change-Id: I37d2c6b9317548922913887940a0164cc2067efb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2054085Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66253}