Whitespace-only strings count as empty too.

This behavior is different from BigInt.parseInt(""),
which throws a SyntaxError.

Bug: v8:6791, v8:6957
Change-Id: I6671c803f3ba83e23c3e0cad81d3af29dba61c9f
Reviewed-on: https://chromium-review.googlesource.com/727301Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48797}