The index file will add a reference to this file, and the replay will
also expect that the frame execution function and the context function
exist, so write the file even if the frame is empty

Bug: angleproject:5939
Change-Id: I265d061fdae51b86131d145116ad1c0f9ca8c93f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2882581
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>