Few more NRT tweaks

[AndyButland]

Here's a few more NRT related amends that I found when using these methods in Forms:

• Changed GetAll() on IDataTypeService to return a non-nullable collection.
• Add a ClearSessionValue to ISessionManager (as you can't now set null values).
• Allowed an IStatefulNotification to have null values in the state
  • I had a use case for this - needing to log when a form is moved between folders, and if the folder is the root then the value was null. Could work around it (e.g. passing an empty value for null), but seemed it was valid to allow null here.
• Removed the obsoleting of the synchronous methods in TreeControllerBase. They were introduced here when the asynchronous methods were introduced, but given most Umbraco services and data access is synchronous, in many cases you won't get any benefit using the asynchronous methods, and will add a bit of complexity/noise. So perhaps they should remain valid alternatives, and you choose which you want to use?
• Aligned nullability of retrieving tree nodes and menus, synchronously or asynchronously (such that we no longer can get null values, always empty collection objects).

Also a question... should we have the following in the .csproj files? Reason being I had a couple of errors caught by the build server (which I guess must have these options on somehow for when it builds) but that I missed locally.

  <Nullable>enable</Nullable>
  <WarningsAsErrors>Nullable</WarningsAsErrors>

[github-actions]

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