This current fails, since {mmap} fails with EINVAL for empty mappings.
The destructor already has special handling for a {nullptr} mapping, so
we can just use {nullptr} for empty files. We get a similar error on
windows, and can fix it the same way.

On order to make presubmit checks happy, we have to skip copyright
checking and checking for terminating newlines for empty files.

R=mlippautz@chromium.org

Change-Id: I2b73da7ff6df72d8bdd40df1fff6422e0a46881e
Reviewed-on: https://chromium-review.googlesource.com/c/1424861Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58970}