[turbofan] Fix startsWith when no search term is passed

This CL fixes the behaviour of String.prototype.startsWith when
undefined is passed as the search term. It also implements a small
shorthand when the search term is empty (according to the spec).

Bug: v8:11977
Change-Id: Iec2aa5f4301fcf444f20d5c1a80d3f634624d6f3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3035089


Commit-Queue: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Auto-Submit: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75763}