Fix build error when git history not fully available Can happen when using repo tool shallow copy or mirror features Return default commit_id instead of empty when history is not avaiable though git. Bug: angleproject:8553 Change-Id: Iddc1f0c3507a14c15292545f07223dd0e666a3f8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5319504 Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>