[autoroll] Handle nil mode and strategy in frontend
[autoroll] Handle nil mode and strategy in frontend

This only happens when a new roller is created and the frontend is
pushed to include its config, but the roller has not yet cycled.

Change-Id: Ia91ded3aae697b9ddfe77be384c79db34417f90d
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/316081
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@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