All standard containers provide a method named {empty} to check whether
the container is empty. On {base::Vector}, that method is named
{is_empty}, while {empty} is an unused factory method for creating an
empty {Vector}.
This CL renames {is_empty} to {empty} and removes the unused factory
method.

R=leszeks@chromium.org

Bug: v8:8834
Change-Id: I686bd07527801fbe783c412bc241221d8ec3660a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1547862
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60584}