[autoroller] Don't bail out when git diff was empty but we already removed some submodules.
[autoroller] Don't bail out when git diff was empty but we already removed some submodules.

In the case that we had a merge conflict, chances are that the merge
conflicts were only because there were submodule changes (because
locally we removed them). Because we have removed all submodules in line
396-400, in that case, the merge would be resolved, and git diff would
give an empty result, therefore, it should not be treated as an error
and autoroller should continue to proceed.

Bug: b/198431779
Change-Id: I4a9c637c75172602c3cf649762f87cfd6c80fe6f
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/580825
Auto-Submit: Xin Li <93465216fd46a440eb04b27d6e009aced8b0c153@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot