These args were marked @Nullable, but the corresponding fields in
AutoValue were non-nullable, so remove @Nullable to make them
consistent.
For pastAssignees, any callers passing null would have already
encountered NPE due to the ImmutableSet#copyOf (and, if not that, the
check in the auto-generated constructor). For hashtags, push the logic
guarding against null into the one caller in
ChangeNotesParser#buildState, which already has similar logic to
construct its arguments.
Change-Id: Idc5123ccab4e1d21f932067d9ffff0f98afa5011
6291a6b18c