By making EmptyIdentifierString (used for failure-mode parsing) point to the
empty_string() we can drop nullptr checks in the preparser. This is similar to
what the parser already does.

Change-Id: I4640f7ae6b24afc8d5275818caed0cca185ca72c
Reviewed-on: https://chromium-review.googlesource.com/c/1488759Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59860}