Minor cleanup: some classes in Torque don't yet have any definitions for
their fields, so it doesn't make sense to emit field layout macros for
those classes.

Bug: v8:7793
Change-Id: Iee38aa3cbe684f4a63329a676e2e94944dc05de1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1925010Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#65054}