This CL fixes a previous change in String.prototype.startsWith which
didn't throw an exception (in the optimized version) when `undefined`
was passed as a receiver and the search string was the empty string.

Bug: chromium:1230260
Change-Id: I835bd409b09b78bf7235c77596f62b588c95611d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3040841
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@{#75818}