Fix byte[] nullability annotations in PersonaStateCallback

[JustArchi]

After recent code edit in my project I started making use of AvatarHash field in PersonaStateCallback. Despite current nullability which states that AvatarHash is non-null, my program has failed numerous times with exception similar to below:

ArchiBot|OnUnhandledException() System.NullReferenceException: Object reference not set to an instance of an object.
   at ArchiBot.Platform.Steam.SteamBot.OnPersonaState(PersonaStateCallback callback)
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__127_1(Object state)
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()

I've confirmed my theory that AvatarHash is null by ensuring that this if executes despite my IDE warning that it's always false:

I'm not sure under what exact circumstance this happens, and if it's warranted/intentional, but as it stands today the nullability is simply wrong and I'm only correcting it to what it should be to begin with. I've noticed that AvatarHash in ClanStateCallback below is correctly declared as byte[]?, therefore this could be simply annotation mistake, as the logic looks similar.

As a side note I've corrected GameDataBlob as it uses the same proto declaration and is likely also wrong.

Thank you in advance for accepting this PR.

/cc @yaakov-h @xPaw