FSA: Default description to empty string

This is a behavioral no-op, since not specifying the description field
currently behaves the same as if the description was specified as the
empty string. See https://github.com/WICG/file-system-access/pull/423

Bug: N/A
Change-Id: Id946d822e514d6633513b48c5bf7af0df8c7155c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4638360


Reviewed-by: Nathan Memmott <memmott@chromium.org>
Commit-Queue: Nathan Memmott <memmott@chromium.org>
Auto-Submit: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180608}