Add nullable annotation to Mvc5 and AspNetCore projects

[StefanOssendorf]

I think this is the first PR which will actually hit real world scenarios. So whoever reviews this please make this thoroughly!

IMHO we should try to get rid of as most as possible maybe nulls like ClientContext, LocalContext and ApplicationContext but that's a discussion we should hold in #1233

[StefanOssendorf]

I'll modify this post unitl the PR is approved and merged with mentionable changes:

• ISessionIdManager now disallows null keys because our underlying implementation can't even handle a null as a key.
• Constructors of services (ApplicationContextManager*) now require dependencies to be provided eventhough the MS DI allows null for missing dependencies.
• Blazor.State.SessionManager now does not allow null as a session value. The old behavior just ignored it without changing the current session.