FrameCapture: Write the frame cpp file even if frame is empty 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>