Noop clear of non-existing attachments. Bug: angleproject:4988 Change-Id: Ib6ff9756ec7ae5aa2b11f4d12932829fe05656d6 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2511369 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>