... and do not artificially add at least one dummy.js file.

Until this CL we used to ensure the existence of at least one 'extra
native' by appending a short dummy.js file if v8_extra_library_files
was empty. This file is uselessly compiled and run at startup.

This CL removes that mechanism along with a minor tweak to handle an
empty natives blob.

Bug: v8:9736
Change-Id: I05bf7c54380e77a9105d5c1a1a76983c692faf60
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1809372
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63868}