A ProvidedDiffChange should be able to use an diff, even if it is empty.
Bug: 372936485
Change-Id: If6bea0063e92c8942aed080e4c26b8d032b90faa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5926311
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Scott Lee <ddoman@chromium.org>