Fix config.Dict.schema_proto.
Fix config.Dict.schema_proto.

Dict.schema_proto currently assumes the value_type is not None.
Fix that assumption.

R=40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org

Change-Id: I574d908b6d2d4fda9e261aedefe176a520445c15
Reviewed-on: https://chromium-review.googlesource.com/1222236
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/recipes-py,luci/recipes-py