External string resources of length {0} have a {nullptr} data since
https://crrev.com/c/1424861.
This CL fixes allocation of an external one-byte string from such a
resource.

R=ishell@chromium.org

Bug: chromium:995108
Change-Id: Ic8ef507d808583c1b529085d990d705058953f03
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1781051Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63543}