Add missing @Nullabe for Account in SubmoduleOp

Account can be null which was causing a 'Guice provision errors' and
preventing the Merge operation from completing. Add @Nullable for
account parameter in constructor.

Change-Id: I17103ede239190840eaffdd8aaae0bb71de3609c
Merge branch 'stable-2.9' into stable-2.10

* stable-2.9:
  Add missing @Nullabe for Account in SubmoduleOp